Add degree_offset option to heurisch(), for testing diff degree bounds

See issue 2606

Conflicts:
	sympy/integrals/heurisch.py
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
Changes to support gmpy 1.13 or later, or gmpy2 2.0.0b4 or later.
Improvements to gmpy and gmpy2 fixes.
codeprinters: fix Rational's printing for code (as fractions of doubles)

S(4)/3 * x -> (4.0/3.0)*x for ccode, not 4*x/3
ccode: print Rational numbers with long double
printing.rst: fix doctest failures for fcode
use is_positive in favor of __ge__

relevant issue: http://code.google.com/p/sympy/issues/detail?id=2832
Fix a logically incorrect example in the expand() docstring

The docstring was supposed to describe how to disable the log hint, which is
applied before the mul hint, but it instead showed how to disable the mul
hint, which does nothing in this case.
adding symbolic z0 test case for factorials
Merge branch 'master' into rationals-printing-in-code
Expanding doc for partial fraction decomposition
Added support for Poly(f, x, y).coeff(x*y)
set full=True in the example
update coeff-related docstrings; special case Poly in coeff