SAD: compound formats are gone
ENH: readers support FileSentinels and open_files is back, baby.
TST: added tests for reader's use of FileSentinel
ENH: writers now understand FileSentinel
TST: added tests for writers handling of FileSentinel
DOC: updated docs to mention FileSentinel
DOC: made changes suggested by @jairideout
Merge pull request #711 from ebolyen/compound-formats-are-dead-long-live-compound-formats

Compound formats are dead long live compound formats
Merge branch 'master' into issue-679
fix broken tests