ENH: as_data_frame method for STC objects
CLN: refactor + standardize data_frame internals
CLN: add pandas checking function
FIX: addresses @mluessies concerns

+ fix test_raw doc
FIX: ImportError / RuntimeError
ENH: what's new
Merge pull request #562 from dengemann/stc_pandas

ENH: pandas as_data_frame method for STC objects