Add support for incremental statistics.

```
Support incremental statistics on any statistics that can be incrementally
collected. The consequence would be that all the statistical information would
not have to be re-fetched each time gitinspector was executed.

Storing a hash from the options given and the calculated statistics should make
it possible to also distinguish if the git history is changed up to a certain
point and statistics need to be re-fetched anyway.

/Adam Waldenberg
```

Original issue reported on code.google.com by `gitinspe...@ejwa.se` on 15 Nov 2013 at 12:50