BUG : made Frame respect creation time metadata

Frame(data, frame_no=frame_no, metadata=md_dict) dropped
md_dict.

 - Added test
 - minor style changes
Merge pull request #76 from danielballan/fix-metadata

TST/FIX: Implement metadata in Frame and add test
prototype for a unified load function for all file types
switch to using class_exts instead of hardcoded list. Raise an error if we can't find a loader
break line in exception, fix typos
Add handler switching based on class_exts to pims.open

add some basic tests of pims.open
Add ffmpeg crud from tests to gitignore