Feature / update repo from payload 

This is an attempt to solve #54 *("long processing for POST payload from github")*

As I don't know any rails unit testing this pull request unfortunately comes without any additional tests. Maybe you can have a look...

However, I tested on Redmine installation and it works well!

The new behavior is as follows:
* if there is a repository in the current project which matches the GitHub repo name in GitHub payload, then only this repo get's updated by the plugin
* if not, all repositories in the current project get updated. That's the current behaviour which leads to long processing times when having multiple repos in one redmine project.

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*this is the first ruby script I write/edit after an lynda.com online course, so any help or tips are very much appreciated!* :smile: 
Could we make it so that `repositories` contain a list of just the repositories to update?

The `index` action would be a whole lot clearer if the repositories-selection was moved elsewhere, ie if `find_repositories` (or some new, equivalent method) would handle the payload-inspection instead of simply returning all repositories and letting the action pick the one repository to update.

Does this make sense?