Support git graph diagrams in mermaid

[raghur]

Hi, this adds support for git graph diagrams (like Gitgraph.js) but within mermaid. It isn't as pretty yet but a lot of the functionality is in place.
In short,

gitGraph :                     
options                        
{"key": "value",               
"nodeWidth": 150               
}                              
end                            
    commit                     
    branch newbranch           
    checkout newbranch         
    commit                     
    commit                     
    checkout master            
    commit                     
    commit                     
    merge newbranch            

produces:

There are some tests as well. Would love to get some feedback and eventually get this merged into mermaid.

[raghur]

@knsv Bump! :)

[knsv]

This is now part of release 6.0.0. Thanks for you good work!