DOC: added more explanation to top level package and added supported objects for each format
DOC: explained reserved kwargs in IO
DOC: explained compound formats
DOC: explained filehandle mutation for each registry operation
DOC: added comments to registry
ENH: empty file is now explicitly checked
DOC/STY: pep8 and note on empty sniffer
DOC: changed some language, thanks @johnchase
DOC: Fixed some language and added unit-testing section, thanks @jairideout
DOC: better introduction by @jairideout
DOC: fixed links
DOC: formatting
Merge pull request #688 from ebolyen/issue630

More documentation for IO