Prompt customizing

#### Various enhancements / features for prompt expansion (issue #3 ):
- make the username red and bold when sudo access is activated + change prompt to # : 

>  %u        includes the current username
>  %U       includes the current username, colored red and bold iff sudo access is activated
>  %$       includes a '$' char or a '#' char iff sudo access is activated (usefull for the prompt ending)

 -  	truncate the directory to the first char *after* the first '/' within the cwd (else it looks like the root)
 - replace the homedir with '~' in prompt expansion
 -        %S     includes the return value of the last executed shell command, colored red and bold iff non-zero
 - all documented in the man page

#### git support in prompt:
 - %g     includes the git branch name iff the current working directory is a git repository
 -  added a prompt expansion option to show whether files have changed since the last git commit

#### technical things: 
-  preprocessing of the prompt color options for max efficiency

Check it out at work :-)

![promtp_features](http://jovanbulck.github.io/jo-shell/wiki-images/jsh_prompt_features.png)