fixes #9 | added support for abreviation's explanation attribute, acronym now maps to abbr.

Created originally on Bitbucket by Russ (Russ W)

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

the explanation attribute is the de facto implementation of abbreviation's expanded value.

<acronym> is no longer a valid html element, so until docutils decides that it has some different functionality it's best to map it to <abbr>.

No tests included as it would require a custom role added to the test suite. If you would like that then say the word and I'll add it.