BUG: interp2d would ignore three of its arguments

Also changed the meaning of copy=False to not *promise* a reference
to be stored. The reference/copy is useless anyway.
BUG: interpolate: fix the bounds checks + default fill_value to extrapolation
DOC: interpolate: mention that bounds errors are ValueErrors
Merge pull request #361 from 'pv/fix-interp2d'

BUG: interp2d would ignore three of its arguments

Implement fill_value, bounds_error, and copy arguments in interpolate.interp2d