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