Add 'acronym' to the government schema

This lets us submit and extract the 'acronym' for Organisations (though there's
nothing stopping anything else using it).

When we're using the field from the index, we can remove the code that fills it
in by processing the title.
Index acronyms and boost them just as we do titles

This is to ensure that organisations continue to rank brilliantly for their
acronym.

Thanks to @fatbusinessman
Merge pull request #125 from alphagov/add_acronym_to_government_schema

Add 'acronym' to the government schema