use boolean indexing via getitem to trigger masking; add inplace keyword to where
in core/frame.py

changed method _getitem_ to use _mask_ directly (e.g. df.mask(df > 0) is equivalent semantically to df[df>0])
  this would be a small API change as before df[df >0] returned a boolean np array

added inplace keyword to _where_ method (to update the dataframe in place, default is NOT to use inplace, and return a new dataframe)

changed method _boolean_set_ to use where and inplace=True (this allows alignment of the passed values and is slightly less strict than the current method)

all tests pass (as well as an added test in boolean frame indexing)

if included in 0.9.1 would be great (sorry for the late addition)
