Improvements to str_cat
closes #11435 
closes #11334.

Modified behavior of str_cat to ignore NaN values in the Series by default.

Added some doc examples to make the behavior with nan more clear, and catch an error condition when a sep is passed instead of an others (array) as an argument and print a more intuitive error message.

This is a retry of PR #12000.

Before, if NaN values were present in the Series, calling `cat()` on it would return NaN, unlike the rest of the `Series.str.` API. This behavior has been amended to ignore NaN values by default.
(#11435)

A new, friendlier ValueError was also added to protect against the mistake of supplying the `sep` as an arg, rather than a kwarg.
(#11334)

``` python

    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ')
    'a b c'

    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ', na_rep='?')
    'a b ? c'

    >>> Series(['a','b',np.nan,'c']).str.cat(' ')
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-3-338c379049e9> in <module>()
    ----> 1 Series(['a','b',np.nan,'c']).str.cat(' ')

    [...]

    ValueError: Did you mean to supply a `sep` keyword?
```

looks good.

some small comments and you have linting errors:

```
linting -> pandas/core
pandas/core/strings.py:4:80: E501 line too long (84 > 79 characters)
linting -> pandas/io
linting -> pandas/stats
linting -> pandas/compat
linting -> pandas/sparse
linting -> pandas/tools
linting -> pandas/tseries
linting -> pandas/tests
pandas/tests/test_strings.py:2463:24: E231 missing whitespace after ','
pandas/tests/test_strings.py:2463:28: E231 missing whitespace after ','
pandas/tests/test_strings.py:2463:32: E231 missing whitespace after ','
pandas/tests/test_strings.py:2471:5: E303 too many blank lines (2)
linting -> pandas/computation
linting -> pandas/util
```
 ping when green.
 @hack-c thanks, I make the flake fixes.
 Hey @jreback so is this all done?
 yep all done. thanks!

just trying to get the 0.18.0RC1 out the door :)
 Ok, great @jreback!  

How can I help? I want to do more :)
 @hack-c great!

just troll issues, you can select the difficulty labels as you want!
