Improve build script.

This will clone the official repo if a user's repo doesn't exist. It will also attempt to check out the branch that CraftBukkit is building off of. This is for those who submit pull requests with a different branch name, such as referencing a Leaky issue. Per @authorblues suggestion, a note in a CONTRIBUTING.md file stating to use a different branch name, with the same across both forks would be a good idea.