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
     added Dtype printing to show on default with a Series
     added convert_dates='coerce' option to convert_objects, to force conversions to datetime64[ns]
     where can upcast integer to float as needed (on inplace ops #2793)
     added fully cythonized support for int8/int16
     no support for float16 (it can exist, but no cython methods for it)

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
     fixes for test_excel on 32-bit
     fixed test_resample_median_bug_1688 I belive
     separated out test_from_records_dictlike
     testing of panel constructors (GH #797)
     where ops now have a full test suite
     allow slightly less sensitive decimal tests for less precise dtypes

BUG: fixed GH #2778, fillna on empty frame causes seg fault
     fixed bug in groupby where types were not being casted to original dtype
     respect the dtype of non-natural numeric (Decimal)
     don't upcast ints/bools to floats (if you say were agging on len, you can get an int)
DOC: added astype conversion examples to whatsnew and docs (dsintro)
     updated RELEASE notes
     whatsnew for 0.10.2
     added upcasting gotchas docs

CLN: updated convert_objects to be more consistent across frame/series
     moved most groupby functions out of algos.pyx to generated.pyx
     fully support cython functions for pad/bfill/take/diff/groupby for float32
     moved more block-like conversion loops from frame.py to internals.py (created apply method)
       (e.g. diff,fillna,where,shift,replace,interpolate,combining), to top-level methods in BlockManager
Merge remote-tracking branch 'jreback/dtypes' into opt-take