convert factorials to gamma-function format
adding factorial test case
be more explicit with the rewrite operation
stripping trailing whitespace
adding negative number test case
only rewrite for positive z0
use is_positive in favor of __ge__

relevant issue: http://code.google.com/p/sympy/issues/detail?id=2832
adding symbolic z0 test case for factorials
Added support for Poly(f, x, y).coeff(x*y)
update coeff-related docstrings; special case Poly in coeff
change Poly.coeff to Poly.monomial_coeff
change monomial_coeff to coeff_monomial
solve: return all independent solutions.

Now solve will return all independent solutions instead of breaking
out at the first solution found.
edit docstrings
solve: Return all independent solutions (underdetermined systems).
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
Incorrect Exception in dsolve()
added tests
added tests
Travis: enable gmpy tests by default