ENH/REF: Additional methods for interpolate

ENH: the interpolate method argument can take more values
for various types of interpolation

REF: Moves Series.interpolate to core/generic. DataFrame gets
interpolate

CLN: clean up interpolate to use blocks

ENH: Add additonal 1-d scipy interpolaters.

DOC: examples for df interpolate and a plot

DOC: release notes

DOC: Scipy links and more expanation

API: Don't use fill_value

BUG: Raise on panels.

API: Raise on non monotonic indecies if it matters

BUG: Raise on only mixed types.

ENH/DOC: Add `spline` interpolation.

DOC: naming consistency