[minor] Refactoring `join`.

• Remove the custom `mrField` – Changed to `ExprName`, like the rest of the code (now that `ExprName` = “value”).
• Rename a couple functions and one is now a `val`.
Add scripts to help with git workflow.

Two new scripts, `wip` and `ready`. The first just creates a new wip
branch with the given name and also a remote branch. The second is much
more complicated and handles both going from a wip branch to a pull
request (the PR is only created automatically if `hub` is installed) as
well as updating a revised ready branch.
Merge pull request #277 from sellout/ready/git-workflow

Add scripts to help with git workflow.
Merge pull request #276 from sellout/ready/join-refactor

[minor] Refactoring `join`.
awesomer effect isolation; rename new test datasets
Merge branch 'master' into ready/104