add quick solution for linear systems of equations

The routines were added to the solvers.py instead of matrices since
it is in the context of solving symbolic matrices that these are
most applicable.

Systems that hung or were very slow solve very quickly now:

https://code.google.com/p/sympy/issues/detail?id=3512
https://code.google.com/p/sympy/issues/detail?id=2015

If simplify=False they solve even faster.