Pull request: add support for per-file custom annotations

```
This change introduces the ability to attach annotation blobs to each entry in
the index. We index all our internal and external repositories at Twitter, and
the annotations are used to add data like commit count, number of import
references, etc. to each file. This is then used for scoring at search time to
improve ranking.

There are a couple of other minor changes useful for using the code as a
library, such as exposing a couple of private functions or,and and adding a
callback system to match.go.

Change is here:
http://code.google.com/r/alec-codesearch-annotations/source/detail?r=b965ca9c2a9
4d464ef541d05f4004368b9c3508b
```

Original issue reported on code.google.com by `a...@swapoff.org` on 29 Aug 2012 at 10:19