Add override_header_style option to data_frame.to_excel()
[Pandas v0.16.0]

The Excel export (`data_frame.to_excel`) currently hard codes cell styles for all header frames.  This pull request allows this style to be overridden by the user.

Don't override the default header style (Default)
`data_frame.to_excel(..... "override_header_style":None)`

Don't style the header at all
`data_frame.to_excel(..... "override_header_style": {} )`

Pass in a custom style
`data_frame.to_excel(..... "override_header_style": { "font" : {"font_color" : "#990000", "bold" : False }, "align" : {"horizontal" : "right"} } )`

There are currently no tests for the to_excel function mentioned here.  (All the other, existing tests pass.)  I'd be happy to write some tests, though it might mean refactoring some functionality out of constructors as **init** is notoriously difficult to mock.  (mock.assert_called_with ?)
