Simple indexes

[IpShot]

Added simple indexes with String, Number and Date types support

[c58]

1. Make all tests passing please
2. Add tests for IndexManager (from https://github.com/IpShot/marsdb/blob/indexes/test/both/IndexManager.test.js)
3. Use indexes in DocumentRetriver for queries like that: {field: 'some_id'}, {field: {$in: [1,2,3]}}. So, i think getMatching(...) must have some query analyzing logic from DocumentRetriver, and DocumentRetiver must get appropriate index from IndexManager and just use it to get some ids for scan. In other words, getMatching must work with plain-object arguments too and DocumentRetriver must delegate query analyzing to getMatching of some index (or retrive all if no index found)

[coveralls]

Coverage increased (+0.3%) to 90.559% when pulling 25805c8 on IpShot:simple_indexes into e3c157e on c58:master.

[coveralls]

Coverage increased (+0.3%) to 90.559% when pulling 4850fee on IpShot:simple_indexes into e3c157e on c58:master.

[coveralls]

Coverage increased (+0.3%) to 90.55% when pulling 8101008 on IpShot:simple_indexes into e3c157e on c58:master.

[coveralls]

Coverage increased (+0.3%) to 90.545% when pulling 690ab60 on IpShot:simple_indexes into e3c157e on c58:master.