CLN: Refactor string methods and add PandasObject

Previous PandasObject becomes PandasContainer. New PandasObject becomes
baseclass for more elements (like Index, Categorical, etc.), moves
string methods to baseclass and subclassing objects need only define
`__unicode__` methods to get all string methods for free (and Py2/3
compatible).
CLN: Remove unnecessary string methods from frame
CLN: Make Categorical inherit from PandasObject
CLN: Remove extraneous __repr__ from io/excel
CLN: Make GroupBy inherit from PandasObject
CLN: Make PyTables unicode safe + add StringMixin
CLN: Make StataMissingValue use StringMixin
ENH: Use StringMixin for addl string methods in stats
CLN/ENH: Make Sparse* into PandasObjects

Plus get all the string methods working...
CLN: Index now a PandasObject + str method cleanup
CLN: Make tseries/index fit with PandasObject.
CLN: Cleanup extraneous str methods from Panel
CLN: Change name of TestPandasObjects --> TestPandasContainer
DOC: New class hierarchy + StringMixin

DOC/CLN: Remove extra whitespace from v0.12.0.txt

DOC: Add PR issue number too
DOC: Fix spelling error
CLN: Use PandasObject in internals + cleanup
CLN: Make Period into a PandasObject + cleanup