Added interface support

I've extended your analyzer to scan for interface implementations and draw attributes from there too.
Attributes are merged such that the implementation of the interface can add extra attributes and they will be picked up.
I've made sure all tests pass but I didn't go to the added trouble of testing the above "merging" of attributes between interface + implementation.

Also, it would be nice if you could just add [DesignedByContract] to the interface only and have it picked up by the implementation(s). Not sure if this is possible from the Post# API.

Anyway, let me know what you think.