Removed plotting functionality from contributions

* R/contributions.r (contributions): Removed plotting functionality
  and related arguments from function.

* R/contributions.r (contribution_by_user): Removed plotting
  functionality and related arguments from function.
Merged functionality of contributions functions

* R/contributions.r (contributions, contributions_by_user): Merged the
  two functions into one 'contributions'. Added one argument 'by' to
  determine what to group by.

* Changed code style to be consistent with other code

* Added GPLv2 header