BitSet codec

[eheimburg]

Adds a codec for java.util.BitSet as an array of longs. I've also added support for the legacy driver's save format, as that was very helpful to me when upgrading from Morphia 1.X. This gives users an automatic conversion path if they're still on 1.6 or earlier.

[evanchooly]

mvn test -Dtest=BitSetCodecTest # -Dmongodb=<some version>

if you don't have a mongodb running, you can pass that -D property to automatically start one.

[eheimburg]

For whatever reason, I can't get the mvn test syntax to work for single tests, but I got all the tests working here, so it's ready for review!