Externalize prompt colors; fix typo; add "help" and "example" functions for git prompt.

1. Externalize the prompt coloring into ``~/.git-prompt-colors.sh`` or, alternatively, into the same directory from which ``gitprompt.sh`` was run.  The file can have a leading period ('.') or not.

2. Remove extra space a few of the prompts strings.

3. Added ``git_prompt_help`` and ``git_prompt_examples`` functions to make it easy for the user to have a description of the git prompt.