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
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.
Merge pull request #1959 from asmeurer/rf

Move custom rf class outside the combsimp definition
Merge pull request #7434 from skirpichev/pypi-badgets

Add PyPI shields to README
manualintegrate can now handle Heaviside(x)

- Integral(Heaviside(x),x) == x*Heaviside(x)
- enable simple substitutions
- add manualintegrate tests for Heaviside
- add to docs for improving manualintegrate

Also, rename "Log" integration commands to Reciprical for
	readability in manualintegrate module