Add support for filtering the whole bibtex collection and for turning @misc to @online

When I want to have some sane bibtex rather than what mendeley produces I want to do this for my whole library. This makes --input-aux optional. It also turns @misc to @online when urls are present as biblatex can do more with that.

The duplicate entries problem is resolved by using the current trunk of pybtex... which turns the exception to a warning.