Add git based index [part 2]

[r0mainK]

The class representing the index. We use dulwich to manage a cached repo that serves as a local copy of the git repo -> dulwich implements clone/pull/commit/push etc.

• fetch_index: clones the remote index, updates it if necessary, and loads index.jsonin the index class attribute (python dict) -> called at init
• remove_model: removes a model from the index and cached repo -> called in delete
• add_model: adds a model to the index and cached repo --> called in publish
• update_readme: removes cached readme and rewrites it -> called in both publish and delete
• initialize_index: destructively resets the index, cache is purged -> used in init
• upload_index: pushed cached index to git, after overwritting index.json with the current value of the class -> used in init, delete and publish
• load_template -> helper function to load a jinja2 template, used both in add_model and update_readme

At init uses regex to check the given remote_repo, username and password args have a correct format, if either is missing there is no auth in the url. Also initializes commit message and calls fetch index.

[r0mainK]

Ok so there were some subtle things about dulwich i did not take into account, so gonna rewrite the init to handle all protocols/domains names, and will let it do the rest, e.g. check repo exists and credentials are correct

[vmarkovtsev]

Cool. Yes, this is the best we can do.

[r0mainK]

@vmarkovtsev fixed tests we can merge this