Added homebrew install documentation to README

Fixes #69
Updated homebrew instructions in README.md
Moved VirtualEnv template to git-prompt-colors.sh, fixed replacement
added helper function git_prompt_color_samples to show a list with all available named colors
Merge branch 'master' of github.com:magicmonty/bash-git-prompt

* 'master' of github.com:magicmonty/bash-git-prompt:
  Updated homebrew instructions in README.md
Ditched the config variable GIT_PROMPT_SHOW_LAST_COMMAND_INDICATOR in favor of a more general approach with a placeholder.
Fixes #76
Fixed a leftover GIT_PROMPT_SHOW_LAST_COMMAND_INDICATOR
Merge with upstream. Updated README