LevelDB backend

After experimenting with the SQLite backend for a while (#50) and finding its performance to be very much lacking during benchmarking (see table below), I had a go at writing another non-distributed backend, this time using [LevelDB](http://code.google.com/p/leveldb/).

It makes use of the [Plyvel](http://plyvel.readthedocs) bindings, which work with Python2/3 and PyPy.

Here are some very unscientific benchmarks I made:

Backend          |  Submit/Read 1000 tasks  |
-------------------|------------------------- |
LevelDB          |  950ms                   |
SQLite (ramdisk) |  1,120ms                 |
SQLite (disk) [1]|  242,200ms (*yep...*)               |
Redis            |  620ms                   |

[1] 7200rpm SATA 3.0Gb/s formatted with ext4

Together with the SQLite backend, this gives the user some great choices, depending on their needs:

- Redis:
  * :+1: Fastest, distributed
  * :-1: Requires external service
- SQLite:
  * :+1: No dependencies, doesn't require an external service
  * :-1: Slow, not distributed
- LevelDB:
  * :+1: Fast, doesn't require an external service
  * :-1: Not distributed