Split functionality of contributions function about git2r

[karthik]

Sounds good. Do you want to do the plotting as a method (with classes) or just have them as two separate functions?

[stewid]

I have implemented the contributions as a generic function with the following three signature methods

1. missing use current working directory. Later we can implement discover repository
2. character use as path to repository
3. S4 class git_repository performs the actual work
   The methods have one argument breaks to handle year, week or day. The methods also have one argument by to determine if to group by commits or by user

I have also started to work on a plot method with signature S4 class git_repository. It also take the two arguments breaks and byAt the moment it only works when plotting the commits over time.

What do you think?

[stewid]

Fixed plotting and updated README