Add local git hooks and an install target

Core developers and frequent contributors can install the hooks by running:

  make setup-hooks

The hooks will:

 - ensure that the sources are amalgamated when committing

 - that the commit message follows the GitHub guidelines

 - append the GitHub guidelines to the commit message template

If the commit message check fails, it will show the user how to
re-edit the previous commit message.

The hooks are implemented as shell scripts.
Merge pull request #131 from cesanta/hooks

Add local git hooks and an install target