ENH: added api_rst_coverage.py
Closes https://github.com/pydata/pandas/issues/8163

How does this look? I wasn't sure how to specify the path to api.rst.
 can u post output?
 Output (with v0.14.1):

```

Class members in api.rst that aren't actual members:
Series.cat
Series.dt
Series.itemsize
Series.nbytes

Class members (other than those beginning with '_') missing from api.rst:
Series.add_prefix
Series.add_suffix
Series.argmax
Series.argmin
Series.as_blocks
Series.as_matrix
Series.at_time
Series.axes
Series.between_time
Series.bfill
Series.blocks
Series.bool
Series.compound
Series.compress
Series.consolidate
Series.convert_objects
Series.data
Series.divide
Series.dot
Series.dtypes
Series.empty
Series.ffill
Series.filter
Series.flags
Series.from_array
Series.ftypes
Series.get_dtype_counts
Series.get_ftype_counts
Series.get_value
Series.get_values
Series.iget
Series.iget_value
Series.imag
Series.index
Series.irow
Series.is_copy
Series.is_time_series
Series.item
Series.items
Series.iterkv
Series.keys
Series.kurtosis
Series.load
Series.mask
Series.multiply
Series.nlargest
Series.nonzero
Series.nsmallest
Series.pop
Series.product
Series.ptp
Series.put
Series.ravel
Series.real
Series.reindex_axis
Series.rename_axis
Series.repeat
Series.reshape
Series.save
Series.set_axis
Series.set_value
Series.slice_shift
Series.squeeze
Series.subtract
Series.swapaxes
Series.to_period
Series.to_timestamp
Series.tolist
Series.transpose
Series.tshift
Series.valid
Series.view
Series.where
Series.xs
DataFrame.as_blocks
DataFrame.at_time
DataFrame.between_time
DataFrame.bfill
DataFrame.blocks
DataFrame.bool
DataFrame.columns
DataFrame.compound
DataFrame.consolidate
DataFrame.divide
DataFrame.dot
DataFrame.empty
DataFrame.ffill
DataFrame.get
DataFrame.get_value
DataFrame.get_values
DataFrame.icol
DataFrame.iget_value
DataFrame.index
DataFrame.interpolate
DataFrame.irow
DataFrame.is_copy
DataFrame.items
DataFrame.iterkv
DataFrame.keys
DataFrame.kurtosis
DataFrame.load
DataFrame.mask
DataFrame.multiply
DataFrame.pivot_table
DataFrame.product
DataFrame.rename_axis
DataFrame.save
DataFrame.set_axis
DataFrame.set_value
DataFrame.slice_shift
DataFrame.squeeze
DataFrame.subtract
DataFrame.swapaxes
DataFrame.to_wide
DataFrame.tshift
DataFrame.where
Panel.align
Panel.as_blocks
Panel.as_matrix
Panel.at_time
Panel.between_time
Panel.bfill
Panel.blocks
Panel.bool
Panel.compound
Panel.consolidate
Panel.convert_objects
Panel.describe
Panel.divide
Panel.empty
Panel.ffill
Panel.fromDict
Panel.get
Panel.get_values
Panel.head
Panel.interpolate
Panel.is_copy
Panel.items
Panel.iterkv
Panel.keys
Panel.kurt
Panel.kurtosis
Panel.load
Panel.mad
Panel.major_axis
Panel.mask
Panel.minor_axis
Panel.multiply
Panel.product
Panel.rename_axis
Panel.replace
Panel.save
Panel.set_axis
Panel.slice_shift
Panel.squeeze
Panel.subtract
Panel.tail
Panel.toLong
Panel.to_dense
Panel.to_long
Panel.to_msgpack
Panel.to_sql
Panel.tshift
Panel.where
Panel4D.abs
Panel4D.add
Panel4D.add_prefix
Panel4D.add_suffix
Panel4D.align
Panel4D.apply
Panel4D.as_blocks
Panel4D.as_matrix
Panel4D.asfreq
Panel4D.at
Panel4D.at_time
Panel4D.between_time
Panel4D.bfill
Panel4D.blocks
Panel4D.bool
Panel4D.clip
Panel4D.clip_lower
Panel4D.clip_upper
Panel4D.compound
Panel4D.conform
Panel4D.consolidate
Panel4D.convert_objects
Panel4D.count
Panel4D.cummax
Panel4D.cummin
Panel4D.cumprod
Panel4D.cumsum
Panel4D.describe
Panel4D.div
Panel4D.divide
Panel4D.drop
Panel4D.dropna
Panel4D.empty
Panel4D.eq
Panel4D.equals
Panel4D.ffill
Panel4D.fillna
Panel4D.filter
Panel4D.first
Panel4D.floordiv
Panel4D.fromDict
Panel4D.from_dict
Panel4D.ge
Panel4D.get
Panel4D.get_value
Panel4D.get_values
Panel4D.groupby
Panel4D.gt
Panel4D.head
Panel4D.iat
Panel4D.iloc
Panel4D.interpolate
Panel4D.is_copy
Panel4D.items
Panel4D.iteritems
Panel4D.iterkv
Panel4D.ix
Panel4D.join
Panel4D.keys
Panel4D.kurt
Panel4D.kurtosis
Panel4D.labels
Panel4D.last
Panel4D.le
Panel4D.load
Panel4D.loc
Panel4D.lt
Panel4D.mad
Panel4D.major_axis
Panel4D.major_xs
Panel4D.mask
Panel4D.max
Panel4D.mean
Panel4D.median
Panel4D.min
Panel4D.minor_axis
Panel4D.minor_xs
Panel4D.mod
Panel4D.mul
Panel4D.multiply
Panel4D.ne
Panel4D.pct_change
Panel4D.pop
Panel4D.pow
Panel4D.prod
Panel4D.product
Panel4D.radd
Panel4D.rdiv
Panel4D.reindex
Panel4D.reindex_axis
Panel4D.reindex_like
Panel4D.rename
Panel4D.rename_axis
Panel4D.replace
Panel4D.resample
Panel4D.rfloordiv
Panel4D.rmod
Panel4D.rmul
Panel4D.rpow
Panel4D.rsub
Panel4D.rtruediv
Panel4D.save
Panel4D.select
Panel4D.sem
Panel4D.set_axis
Panel4D.set_value
Panel4D.shift
Panel4D.skew
Panel4D.slice_shift
Panel4D.sort_index
Panel4D.squeeze
Panel4D.std
Panel4D.sub
Panel4D.subtract
Panel4D.sum
Panel4D.swapaxes
Panel4D.swaplevel
Panel4D.tail
Panel4D.take
Panel4D.toLong
Panel4D.to_clipboard
Panel4D.to_dense
Panel4D.to_excel
Panel4D.to_frame
Panel4D.to_hdf
Panel4D.to_json
Panel4D.to_long
Panel4D.to_msgpack
Panel4D.to_pickle
Panel4D.to_sparse
Panel4D.to_sql
Panel4D.transpose
Panel4D.truediv
Panel4D.truncate
Panel4D.tshift
Panel4D.tz_convert
Panel4D.tz_localize
Panel4D.update
Panel4D.var
Panel4D.where
Panel4D.xs
```
 @jorisvandenbossche what do you think on this?
 @seth-p can you add a note in using this in `contributing.rst`? thxs.
 Added a note about `scripts/api_rst_coverage.py` to `contributing.rst`. Let me know if it looks ok.
 @jorisvandenbossche ?
 @jorisvandenbossche look ok?
 I have some ideas to improve it, but won't be able to do that right now, so let's merge this as a good start!

Merged via https://github.com/pydata/pandas/commit/83d4dfd3e3ba20693605ab3e3129e2b7ed42b785
