Refactored sympy.core.relational.

Broke out logic from the Relational class into subclasses.  Simplified the
logic without modifying it.  Eliminated the helper functions, replacing with
aliases to the relevant classes.  Added docstrings for Relational, Equality,
and Unequality.
Refactored relational: edits as per @asmeurer's suggestions.
Refactored relational: clarified Eq and Ne vs. == and !=.
resolve conflicts with merge