Add BoltDB backend

There's a flurry of discussion about what makes a good local persistent backend. LevelDB was a choice, and a good one, but there's a lot of options out there. I've been weighing a few of them and I'm coming around to [Bolt](http://github.com/boltdb/bolt) as a pretty good one. 

For one, it improves our ACID story considerably. It has real read/write transactions and a nice API. That alone is a selling point for me. It writes relatively quickly (or at least, certainly not more slowly) than LevelDB and is well-supported. It does have some disk overhead on small stores (eg, 600Mb for the movies set against ~225Mb) but that starts to even out on larger ones. The corollary is that its memory usage would likewise be higher.

But then how does it do on queries?
```
$ benchx leveldbpr.txt boltpr.txt
benchmark                                   old ns/op      new ns/op     delta          mult
BenchmarkNamePredicate                      3807236        1409078       -62.99%        -2.70x
BenchmarkLargeSetsNoIntersection            925544265      187675217     -79.72%        -4.93x
BenchmarkVeryLargeSetsSmallIntersection     1306140818     471821173     -63.88%        -2.77x
BenchmarkHelplessContainsChecker            1437189511     367282585     -74.44%        -3.91x
BenchmarkNetAndSpeed                        80003659       25535952      -68.08%        -3.13x
BenchmarkKeanuAndNet                        37146352       16469829      -55.66%        -2.26x
BenchmarkKeanuAndSpeed                      60546303       31867784      -47.37%        -1.90x
BenchmarkKeanuOther                         465388076      175475491     -62.29%        -2.65x
BenchmarkKeanuBullockOther                  917129328      229777168     -74.95%        -3.99x
```
(benchx is a simple awk script of mine for the last column)

In short, it's roughly 3x faster across the board.  

I'm happy to keep testing, but it's at least time to share and get comments.

(Incidentally, this would fix #9 (number nine... number nine....))
Use short declaration for old_size and old_horizon.