Capitalize words in title between curly brackets

(following up on issue #2 and issue #3)

RE [this comment](https://github.com/ptigas/bibpy/pull/3#issuecomment-15003113): that's interesting; I'm learning a lot about BibTex! 

How about this change? Now it searches for all paired curly brackets in the title, and capitalizes the words between paired brackets. I did it in such a way that it should avoid interpreting apostrophes as breaks between words, but it looks like there is something else in your code breaking words at apostrophes?  Didn't look too hard for it.

You might want to move capitalize() (line 240) to somewhere else.