add derivatives to bivariate splines

These changes add a fortran routine to return derivatives at arbitrary
points and includes derivative evaluation for Bivariate splines. Keyword
'grid' is added to the Bivariate spline evaluation to remove the
necessity of the 'ev' method for arbitrary point evaluation. These
changes have been tested as working with the 0.12 branch.
updated to mater branch

merged the changes to the master branch as requested
fixed a few typos
*sigh* another typo
added test functions for derivatives

the test functions should check the added derivative capability as well
as the added 'grid' keyword
integrate: fix issues in the __call__ interface in *BivariateSpline

- make __call__(..., grid=False) to broadcast
- add more tests
- reduce code duplication
- remove automatic grid sorting
interpolate: fix shape of empty returns
interpolate: blank out the mth argument, it does nothing
Merge pull request #2809 from ktritz/master

ENH: interpolate: add derivative evaluation to bivariate splines

These changes add a fortran routine to return derivatives at arbitrary points
and includes derivative evaluation for Bivariate splines. Keyword 'grid' is
added to the Bivariate spline evaluation to remove the necessity of the 'ev'
method for arbitrary point evaluation.