Added support for Mercurial repositories. Works in the same way as GIT; themes need to use functions defined in lib/hg.zsh.

Currently uses git theme variables ($ZSH_THEME_GIT_PROMPT_*). Remove assignments at the top of hg.zsh and define them in themes to change.
Cleaned lib/hg.zsh of references to git.

Theme now sets necessary vars for hg info on prompts.
Merge remote-tracking branch 'mathew-hall/master'