Add procelain to manage cloning, and committing to repos

[shonfeder]

Currently users have to:

1. clone a contract repo
2. cd into the repo
3. compile and sign
4. stage changes
5. commit changes
6. push back to repo

It would provide a cleaner UI if we wrap (1) - (2) and (5) - (6) in some porcelain. We could download the repositories into the ~/.neat directory and make the list of signed contracts available. Registering a repo could just be a matter of pasting the giturl.

[shonfeder]

I'd be happy to help implement this.

[shonfeder]

Labeled as high priority, based on its direct relevance for this need from the one-pager alignment doc:

Ability for both technical and non-technical counterparties to sign their contract using the tool or an external integration.

on the grounds that having to be a competent git user shouldn't be a prerequisite for compiling the contract and making changes.

[thanethomson]

Now available in v0.2 (currently on branch prototype/v2, soon to be merged to master).