Add Dataset.dropna() method
Add DataArray.dropna()

Fixes #58
speedup internal creation of dataarrays from datasets
Document dropna
change the dropna argument vars to subset
work around numpy crash with size 0 arrays

here's the numpy issue: https://github.com/numpy/numpy/issues/5195
Copy dropna logic from pandas.DataFrame
Add pandas license
Add unit tests for Dataset.dropna how="all" and thresh
Add how=all and thresh options to DataArray.dropna
Validate exceptions in dropna
Merge pull request #252 from xray/dropna

Add dropna method to Dataset and DataArray