Add option to perform a standard fast-forward merge when accepting merge requests

Adds a new checkbox next to the "Remove source-branch" checkbox that when checked will perform a standard fast-forward merge when accepting the merge request. A new config entry is also added allowing to set the default state of the checkbox.

![fast-forward merge](https://f.cloud.github.com/assets/306980/1640634/8f46fa5e-583e-11e3-84ba-e1399c1310d5.png)