[WIP] Compound Indexes

Fixes #761, depends on https://github.com/doctrine/mongodb/pull/154

TODO:
- [x] adjust index checking
- [x] tests (without getUnindexedFields thing yet)
- [ ] change getUnindexedFields method in Query\Query
- [ ] document CompoundIndex
- [ ] document allowLessEfficientIndexes in @Document annotation
- [ ] document allowLessEfficientIndexes in Query\Builder

cc @dwieeb