Add a new "Used By" analyzer node on types

The "Used By" analyzer node on types, is very handy to quickly find type references.

This PR is an implementation for that in ILSpy:
![ilspy](https://cloud.githubusercontent.com/assets/638167/5505689/8928b5e2-8793-11e4-8d34-8ccf9e995fff.PNG)

Here is the Reflector output on the same assembly list:
![reflector](https://cloud.githubusercontent.com/assets/638167/5505691/8ee55ecc-8793-11e4-9d3b-c47d769cb180.PNG)

Seb