A fairly bug-free version of the search

[hc5]

Really really really sorry about the big commit but I couldn't think of a way to split it up logically, the good thing is that it doesn't modify any old behavior and barely changes existing code so it's quite easy to merge in.
Can't really find any bugs right now, but things TODO:

• pagination of the results, currently only returns the first 10, but the offset parameter is coded in already, so it's easy to add.
• Nicer preview text
• Nicer search page in general
• Better way of displaying the commits. They are already sorted by relevance, so it's just a GUI problem
• Speed. I'm satisfactory with the search speed for now, but the indexing speed is still sort of slow

To "install" you have to syncdb then ./manage.py buildindex. This indexes all the existing documents. Subsequent document editions/creations will be indexed automatically