Minor improvement: Change to_excel "sheet_name" to "sheetname"
pd.read_excel() takes "sheetname" as an argument.
pd.DataFrame.to_excel() takes "sheet_name" as an argument.

For consistency, I think one ought to be changed to match the other.
On my windows installation of pandas 0.16.2, I modified to_excel to take sheetname rather than sheet_name.  All I had to do was locate frame.py within the core directory.  to_excel() is defined here, and sheet_name is written twice in the function and once in the docstring.  

Very new to github so if ideas like these are more appropriately filed through PRs I apologize.
