Add an empty line after all section titles.

This commit fixes the layout of section titles throughout the whole
``diagram_drawing.py`` file.
rsolve: Simplify the returned result.

There is no need to reintroduce the common denominator that was
previously identified, as it only makes the result more complicated.
combsimp: Simplify FallingFactorial and RisingFactorial.

In combsimp, rewrite expressions using gamma whenever possible,
thus allowing to simplify FallingFactorial and RisingFactorial.
rsolve: Handle recursive problems for which y(0) = 0.

If there is a nonnegative root in the denominator of the ratio
y(n + 1)/y(n), this indicates that the term y(n_root) is zero.
One should then solve the recurrence relation starting at the
following term, y(n_root + 1). This fixes issue 2408.
rsolve: Bring test coverage to 100%.
rsolve_poly: Fix python 3 failure.
Try to match against the identity in AssocOp.match.

If we are trying to match a Mul, the code would iterate over its args and test
all possible matching combination. However this misses cases were a Wild would
match on the mutiplicative identity (1). This commit includes the identity (1)
as a possible matching candidate. This fixes issues 674 and 2069.

Note that something like this was already done for Add, but the code used
expr.as_independent instead of the additive identity (0). Of course, this
expr.as_independent would either return the additive constant of the Add, or
simply return zero. The additive constant will already included as part of
expr.make_args. Zero must be added explicitly as a possible match candidate.
Update tests for match.
Update tests for noncommutative.
Merge pull request #1529 from jrioux/solvers

Recursive solver issues
Merge pull request #1523 from jrioux/tests

Failing python 3 pretty printing tests
Merge pull request #1517 from jrioux/tests

Failing python 3 pretty printing tests
Merge pull request #1518 from flacjacket/notebook_v3_update

Update IPython notebooks to version 3 (IPython 0.13) for 0.7.2 release
Merge pull request #1520 from gilbertgede/mechanics_latex

Fixed Minor Bug in physics.mechanics
Merge pull request #1515 from jrioux/simplify

Simplify with noncommutative parts and match issues.
Merge branch 'master' into integration3

This doesn't fix any of the errors that come up as a result of the merge,
except those that were specifically related to the merge, and those that
prevented sympy from importing.  Fixes will come in upcoming commits.

Conflicts:
	bin/coverage_doctest.py
	sympy/core/tests/test_expr.py
	sympy/core/tests/test_subs.py
	sympy/functions/elementary/exponential.py
	sympy/integrals/__init__.py
	sympy/integrals/integrals.py
	sympy/integrals/risch.py
	sympy/integrals/tests/test_integrals.py
	sympy/integrals/tests/test_rationaltools.py
	sympy/integrals/tests/test_risch.py
	sympy/matrices/matrices.py
	sympy/polys/domains/expressiondomain.py
	sympy/polys/domains/realdomain.py
	sympy/polys/euclidtools.py
	sympy/polys/polyclasses.py
	sympy/polys/polyfuncs.py
	sympy/polys/polytools.py
	sympy/polys/tests/test_densetools.py
fix doc errors
Fix Sphinx docs to build without errors
permutations: +/- on basis of rank

Partitions and Prufer add on the basis of rank and this seems to
be a better idea than adding the elements of the inversion code
together.

rank(i) now gives ith permutation

modify which form of the commutator is returned
named_groups: add the 3 polyhedral groups