API: consistent float_format specification
For now just some observations:
- The docstring of different funtions about `float_format`:
  - the documentation of [`to_csv`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_csv.html) (as well as `to_excel`) states: `string, Format string for floating point numbers`  
    - Example of this usage: `df.to_csv(float_format='%.3f')`
  - The docstring of [`to_string`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_string.html) (as well as `to_html`) says: `one-parameter function, Formatter function to apply to columns’ elements if they are floats`
    - Example of this usage: `df.to_string(float_format='{:.3f}'.format)`
  - The option `display.float_format` also accepts a callable following the [docs](http://pandas.pydata.org/pandas-docs/stable/options.html?highlight=float_format#list-of-options)
- The narrative docs are consistent with that:
  - http://pandas.pydata.org/pandas-docs/stable/io.html#writing-to-csv-format
  - http://pandas.pydata.org/pandas-docs/stable/io.html#writing-a-formatted-string

Question:
- Should we make this consistent? Eg that all methods can accept both a format string as a format function?
- Should we make the usage of `'{:.3f}'.format)` more convenient?

There are a lot of different issues already about formatting, but I didn't find anything about this in specific.
