Include tests in release source tarball

A test suite is a very useful thing, it would be great to include it in the pypi release tarball. That way people who download and build the release can run the tests to ensure the library is working.

Linux distributions, such as Debian, base their packages of Python modules on the pypi release. This means the test suite can be run when building a Debian package, it should catch mistakes in the packaging or errors in dependencies.

I've written some more on this topic on the Debian Python mailing list.

https://lists.debian.org/debian-python/2016/04/msg00074.html

Add test for elif

precedence for `await` and `async for`

Lib/test/test_coroutines.py in the Python 3.5 codebase was observed to
not successfully roundtrip under astor.rtrip due to an await expression
of an await expression (the offending coroutine function has been
excerpted in the `test_double_await` test method introduced in this
commit). The `await` operator has very high precedence, greater than
that of exponentiation, as documented at
https://docs.python.org/3.5/reference/expressions.html#operator-precedence.

Without this, the call to set_precedence in visit_For calls
get_op_precedence, which raises a KeyError.

Add information about other codegen derivatives

Stop triple quoting strings embedded in function calls

pm_develop contains all the work I have been doing over the last couple of weeks

All cleanup changes had been made previously, but were
broken out into a separate branch to make it easier to
view them in isolation

Conflicts:
	astor/code_gen.py

Manually cherry-picked from pretty-print branch to introduce in logical order.

Manually cherry-picked from pretty-print branch to introduce in logical order.

Conflicts:
	tests/test_code_gen.py

  - Test with all available libraries, plus automatically
    generated permutations.

- add some more automated cases
- add some comprehensive (slooow) tests