Add support for typedefs

Typedefs are hyperlinked, both at the definition and at any references.
Searches using type: and type-ref: now find typedefs.
Merge branch 'or-filter' of github.com:abbeyj/dxr into typedefs
Use UnionFilter for handling type: and type-ref: queries.
Merge branch 'testing' of https://github.com/mozilla/dxr into typedefs
Merge support for typedefs: searching and hyperlinking. Closes #73.