Implements a reverse_map() method for JordanWignerMapper().

[MarcoBarroca]

Summary

This implements a reverse_map() method in the JordanWignerMapper(). It allows users to go back from qubit operators into fermionic operators. Completes issue #255.

Also included a unit test.

Details and comments

Instead of using the implementation linked in issue #255 I modified it to use the FermionicOp object for the mapping. Seemed like it would be easier for anyone to understand the code. Both implementations are equivalent.