get_effective_cell for getting the contents of Excel cell when the cell is merged
Closes #4672

is this just a useful method? (e.g. you are not using it anywhere)

tests?
 @jreback https://github.com/cancan101/pandas/commit/e82bfa474d856092ffff6954355d4f8d432bb47d Has tests for the new method.
 @cancan101 oh..ok

can you elaborate on the utility of this though?

it SEEMS useful...but what is the actual usecase?
 Sure. There are plenty of Excel documents where there are multiple columns but some columns share one header cell. A given header cell will apply to multiple columns (think `MultiIndex`). If I want to read the value of a given header cell when there is the potential the cell is merged, I would use this new method.

The classic example that I see is in SEC filing where you end up with:

```
3 months ended:             6 months ended:
1/1/2012    1/1/2011        1/1/2012    1/1/2011
```
 @jreback The idea being that a given cell acts as a header for multiple columns
 Can you add a docstring to the method to explain this.

To me what you're saying seems a bit magical, so example in docstring would be good. Also, why the name "effective" cell, is this standard?
 @hayd I can definitely write a docstring. If you have something better than effective, I can certainly change to that. `effective` sounded reasonable to me.
 Not sure tbh, but there may be an Excel (or more general) term for this. If so we should use it _(I'm still not 100% what this is, so I don' think I'm the best person to google it to see if the term exists :) )_.
 I'd like to close this in favor of a more general way of handling merged cells [at least at the top of columns]. E.g. if you had something like this in the header:

```
_________________________________
|Foo        |  Bar               |
|________________________________|
| A | B | C | D | E | F | G | H  |
|________________________________|
```

This would become  a set of columns that are a `MultiIndex` (and equivalent thing for rows). And the API would just be that you pass in a list/integer for number of rows to consider the "header" or list/integer for number of columns to consider the "index" and then pandas would intelligently handle from there. [so the sum total of the API change would be, potentially, two additional keyword arguments].  Whatever we decide as convention here could also be used for converting DataFrames with MultiIndex into excel files.

If any of this already exists, then that's great.
 and pandas might use something like this function to determine if there are merged cells and, if so, how they should be handled.
 @jtratner I like your suggestion of converting the headings into a `MultiIndex` Whatever solution we come up with for parsing Excel files should also work for HTML tables which I have seen to have the same format.

FWIW, I did find this forum about the issue for Excel files: http://answers.microsoft.com/en-us/office/forum/office_2007-excel/unmerge-cells-and-copy-the-content-in-each/49f46676-e318-4d33-8cac-7c6302214534
 http://pandas.pydata.org/pandas-docs/dev/io.html#reading-columns-with-a-multiindex

already for csv, basically `header=[0,1]`
 @jreback What about HTML?
 I think `HTML` is quite tricky to do this (I am not sure that it accepts a `header` option at all), that said if you could specify a way to do it creating the mi is not hard, but why wouldn't you do it after anyhow. HTML is not very regular.
 The HTML that I am thinking about is regular enough. The rows all have the same number of "columns" and the html uses colspan where the Excel uses merged cells. See for example: http://www.sec.gov/Archives/edgar/data/47217/000104746913006802/a2215416z10-q.htm#CCSCI
 I take it back, I forgot that `read_html` takes a `header` argument. So it would be reasonable for it to deal with a list I suppose (to return a mi for the columns).
 @jreback what is an "mi" ?
 multi-index
 closing in favor of #4682 / #4679 
 @cancan101 yeah, I agree, it's very regular and basically equivalent [i.e., here's a set of cells, some of them have widths and heights]. If you convert to repeating, it's exactly the same as what csv reader would do with them.
 @cancan101 and if it can't make sense of it [i.e., doesn't seem regular], could just fail quickly and make the user munge themselves.
