Move versioning info to CONTRIBUTING.md

Also, major version bumps are for tracking backwards compatibility, not
milestones.
Remove dependency requirements from README.md
Test: make xbrl.clean_concepts() expect dict

Instead of taking individual elements, xbrl.clean_concepts() should take
a dictionary of elements created using xbrl.open_linkbases().
Add tests for xbrl.open_linkbase()

Function should accept an instance document filename and a list of
linkbases to open. A dictionary should be returned containing the
filenames of each linkbase, along with an lxml ElementTree and root
Element.
{"xsd": {
"filename": "/home/user/abc-20130331.xsd",
"tree": etree.ElementTree(),
"root": etree.Element()
}, ...}
Add xbrl.open_linkbases()

Function abstracts out the linkbase opening functionality which is
duplicated all over the place.
Rewrite xbrl.clean_concepts() to accept dict
Open all linkbases using xbrl.open_linkbases()