ENH: add .resample(..).interpolate() #12925
- [x] closes #12925
- [X] tests added / passed in pandas/tseries/tests/test_resample.py: Base.test_resample_interpolate()

These tests simply check that df.resample(...).mean().interpolate() gives the same result than df.resample(...).interpolate()
- [X] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

-> Edit: 24/04/2016 20:25
