Move custom rf class outside the combsimp definition

This fixes, or rather, works around, a bizarre caching issue involving classes
defined inside functions.  See the discussion on pull request #1710.
change rf to _rf in simplify
Merge pull request #58 from smichr/rf

change rf to _rf in simplify
Drop ancient oseries method
Drop _hashable_content (inherit from Basic)
Change dict.has_key(key) to key in dict
pep8: E211 whitespace before '('
pep8: E703 statement ends with a semicolon
UnXFAIL test_sympy__matrices__expressions__matexpr__ZeroMatrix
Add test for #7231, fixed with pr#2846, fixes #7231
UnXFAIL test_as_coeff_add
Change remaining iteritems to items
Enable code for _EXPAND_INTS=True in cos._eval_rewrite_as_sqrt

See pr #2645 and issue #7210.

The "_EXPAND_INTS" flag allows rewritting cosine (and sin) in cases
where the formula isn't closed over the rationals, e.g.:

    >>> cos(pi/14).rewrite(sqrt)
    sqrt(cos(pi/7)/2 + 1/2)
Removed unused and untested code in polycontext.py, fix #7210 (see also pr #2645)
Return oo for harmonic(oo), this fixes #4668

This value is consistent with this reference from the docstring:
http://functions.wolfram.com/GammaBetaErf/HarmonicNumber/
Merge pull request #7420 from skirpichev/4668-harmonic-at-oo

Return oo for harmonic(oo), this fixes #4668
Merge pull request #7306 from skirpichev/misc

Misc fixes
Added regression test for issue 7425.

Make sure lambdify can handle Python reserved keywords as symbol names.
Add PyPI shields to README
Merge pull request #7427 from moorepants/issue-7425

Added regression test for issue 7425.