[WIP] Add git aliases.

- [ ] mkbranch = `git checkout -b`
- [ ] empty = `git commit --allow-empty`
Enable `git mkbranch` alias to mean `git checkout -b [branch name]`.
Enable `git empty` alias to mean `git commit --allow-empty -m "[WIP] " -e`.
Merge pull request #16 from kazu634/gitconfig_20141019

Add git aliases:

- mkbranch alias, which means "git checkout -b"
- empty alias, which means "git commit --allow-emtpy"