Partial support for multi-indexes with an object model (#174)

Only GetAll supports multi-indexes; EqJoin also should for sure, and I'm
not sure if any other operations are supported on multi-indexes.
Add support for EqJoin on IMultiIndex
Introduce a common base interface for indexes, IBaseIndex

Reduces duplication in query operations that support both multi-indexes
and single-indexes without any difference in type signature.
Merge branch 'master' into issue_174
Merge pull request #174 from mfenniak/issue_174

Support multi-indexes in IndexDefine