Configure hooks and aliases for git commit signing

Per a conversation [1] on the bitcoin-development mailing list:

 - Update the global commit alias `ci` to include -S such that all
   commits are signed by default.

 - Add a new global alias mapping `logsig` to `log --show-signatures`

 - Add a .git_templates directory with post-commit and post-merge
   hooks that will be added by default to any new (or already-existing)
   repository upon running `git init`. See [2] for details.

[1]: http://tinyurl.com/ov2fzjl
[2]: http://stackoverflow.com/a/8842663