ENH/BUG/DOC: allow propgation and coexistance of numeric dtypes
             closes GH #622
DOC: whatsnew for 0.10.2
ENH: construction of multi numeric dtypes with other types in a dict
     inclucing some subtle 0 ndim ndarray construction issues
     validated get_numeric_data returns correct dtypes
     added blocks attribute (and as_blocks()) method that returns a dict of dtype -> homogeneous Frame to DataFrame
     added keyword 'raise_on_error' to astype, which can be set to false to exluded non-numeric columns
     added keyword 'copy' to astype, which by default returns a data copy, otherwise a view
BUG: fixed bug in 1-len passed datetime64 like object construction (now all datetime series are constructed as M8[ns])
     fixed up tests from from_records to use record arrays directly
       NOTE: using tuples will remove dtype info from the input stream (using a record array is ok though!)
     fixed merging to correctly merge on multiple dtypes with blocks (e.g. float64 and float32 in other merger)