Pandas has deprecated the TimeGrouper

[jhamman]

Code Sample, a copy-pastable example if possible

da.resample(time='MS').sum('time')

Problem description

Pandas has deprecated the TimeGrouper class (pandas-dev/pandas#16747) and that warning has started popping out during xarray resample operations. We can make this go away quite easily. (I'll submit a PR shortly).

Output of xr.show_versions()

In [2]: xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.9.6-75-g246c352
pandas: 0.21.0
numpy: 1.13.3
scipy: 0.19.1
netCDF4: 1.3.0
h5netcdf: 0.5.0
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.15.4
matplotlib: 2.0.2
cartopy: 0.15.1
seaborn: 0.8.1
setuptools: 33.1.0.post20170122
pip: 9.0.1
conda: None
pytest: 3.2.3
IPython: 5.2.2
sphinx: 1.6.3