Add Makefile for cleaning up
Permit None gh argument to GitRepository

Not having a github.Github object prevents remote
invocation, but also significantly speeds up
execution. This may need further refactoring
such as a subclass, etc for other commands.
Add call_no_wait method

As seen before, calling popen.wait() when
stdout or stderr is set leads to processes
hanging.
Minor refactoring in prep for UnrebasedPrs
Add UnrebasedPRs command

Command is used to compare two branches and look for
and merge commits which match the Github-style pattern.
Then, the git notes under ref "see_also/BRANCH" are
consulted to see if the PR has a rebase-pointer. If yes,
then it is ignored. Otherwise, the PR is printed to either
stdout or to a file. The two files from each invocation can
be used (via the --parse flag) to add notes so that they will
be filtered on subsequent invocations.
Allow n/a entries for PRs

For PRs which are intended to only be on one branch
(like bumping the version number), there needs to be
a way to set the see_also note to "n/a". That's now
done by placing "n/a" on a line to match the one-off
PR.
Handle non-merge commits as well
Merge pull request #68 from joshmoore/unrebased-prs

UnrebasedPRs command