closes 2775
Cleanup Mul._eval_is_imaginary
added documentation for discrete RV types
Relational: return BooleanAtoms instead of bools.

Fixed tests in several locations to check for the new results.  Corrected
several areas where Python bools were expected; these areas should now handle
Python bools and SymPy BooleanAtoms equally.
Merge pull request #2844 from Tempel/relational_STrue

Relational: return BooleanAtoms instead of bools.
Merge pull request #2792 from m777z/patch-1

added documentation for discrete RV types
Add m777z to the AUTHORS/aboutus. Welcome to SymPy!
Merge pull request #2776 from skirpichev/gh2775

Fix github's issue 2775