Added 'mgcv' compatible stateful transforms

New 'cr/cs', 'cc' and 'te' stateful transforms matching what is found
in the 'mgcv' R package.
Added missing "cc" in __all__ symbols.
Added TODOs for TE class
Completed stateful transforms classes

Stateful transforms 'cr'/'cs', 'cc' and 'te' contains now true
stateful_transform functionality.
Reworked all code...

- Reworked all code: pep8 compliance, added proper docstrings
- Added special treatment for values outside knots range for 'cr/cs'
- Reworked 'te' api to have full control on constraints
application/derivation
- Added basic tests, to be completed with full scale R comparisons
Corrected CubicRegressionSpline bug with pandas.Series
Added full scale R compat tests for 'cr', 'cs' and 'cc'.
Added tests for 'te' with R comparisons
Minor raw R results alignment
ignore - only to trigger travis
Ignore warnings in tests
Knots computation using explicit lower and upper bounds.

- New knots computation using lower and upper bounds and number of
requested inner knots.
- Modified copyright
- Removed hard dependency on scipy
- Corrected doc's 'versionadded'
Reworked 'cr', 'cc' and 'te' API

+ Deleted 'cs'
Quick doc fix, waiting for major update
Improved code coverage of mgcv_cubic_splines
Switch to more DRYish version numbering
[travis] Work around pydata/pandas#7284, plus some general tidying
[travis] Apparently pip install -q is *too* quiet and makes travis give up
Added dedicated doc page for spline regression
Doc improvements after last code review