ENH: allow propgation and coexistance of numeric dtypes (closes GH #622)
     construction of multi numeric dtypes with other types in a dict
     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
     fixed merging to correctly merge on multiple dtypes with blocks (e.g. float64 and float32 in other merger)
     changed implementation of get_dtype_counts() to use .blocks
     revised DataFrame.convert_objects to use blocks to be more efficient

TST: fixed test in test_from_records_sequencelike (dict orders can be different on different arch!)
       NOTE: using tuples will remove dtype info from the input stream (using a record array is ok though!)
     test updates for merging (multi-dtypes)
     added tests for replace (but skipped for now, algos not set for float32/16)
     tests for astype and convert in internals
DOC: added astype conversion examples to whatsnew and docs (dsintro)
     updated RELEASE notes
     whatsnew for 0.10.2