solve: handle Matrix equations.

If we have a Matrix equation Eq(A*B, C), it is transformed into a
system [A*B - C]. We need to iterate over the elements of the matrix
A*B - C again, turning the Matrix equation into a system of equations.

Fixes issue 771: Matrix equations
http://code.google.com/p/sympy/issues/detail?id=771
solve: Test multiple matrix equations.
Merge pull request #1802 from jrioux/matrices

solve: handle Matrix equations.