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
solve: Test multiple matrix equations.
Poly.coeff is NotImplemented
Fixed Issue 3576
Update sympy/core/symbol.py

Indentations are corrected.
Update sympy/core/tests/test_symbol.py

Test Cases for different types of Parentheses examples have been added.
symbols allows escaping of ,; and space

Also, only one layer of surrounding parentheses are removed from
ranges, so if they are doubled, one pair will remain:

symbols('x((1:3))') -> (x(1), x(2))
Add Akshit Agarwal to AUTHORS/aboutus. Welcome to SymPy!
use string.ascii_letters for Python 3
Merge pull request #1802 from jrioux/matrices

solve: handle Matrix equations.
Merge pull request #1790 from jrioux/solvers

solve: return all independent solutions.
Try to fix Sphinx error
Merge pull request #1812 from smichr/poly-coeffs

Poly coeffs