Adding support for Snow hooks

[daniellanner]

Similar to git, SnowFS offers a default directory for adding custom hooks to be executed upon calling repository commands. This directory is located at $PROJECTDIR/.snow/hooks.

Typical git hooks are scripts, named and titularly executed as:

• applypatch-msg
• commit-msg
• fsmotir-watchman
• post-commit
• post-update
• pre-applypatch
• pre-commit
• pre-merge
• pre-merge-commit
• prepare-commit-msg
• pre-push
• pre-rebase
• pre-receive
• update

Currently these files are not executed when performing the matched action.

At least a subset of the above directives could be implemented to be called when the appropriate action is executed. Example implementations can be existing git hooks.

[sebastianrath]

I just leave the following comments and ideas here. For Unix-based systems, it seems to be a no-brainer to implement it. A platform-independent solution is a little bit more tricky. Things to clarify:

1. platform-independent she-bang support
2. I never liked, that Git hooks are not part of the cloning process and must be separately installed.
   If SnowFS will support remote clone/push, I would like to plan ahead for this feature