"ghprbPullLink" - Added capturing the link to original Pull-Request so that the Jenkins Job can "link-back" to the Pull-Request on GitHub that actually triggered the Job in the first place

"ghprbSourceBranch" - Added capturing "source-branch" name in a Pull-Request (since it would contains information like Ticket Number which is essential for integration with Tracking Systems (like JIRA, RALLY, BUGZILLA etc…)

Sample Source-Branch name "Bug1234-nirav-fix-issue-abcd" this allows to update the ticket with ID-Bug1234 stating that a PR was opened for this ticket
Merge branch 'master' of git://github.com/janinko/ghprb into branch 'master' of git://github.com/niravmsh/ghprb

Conflicts:
	src/main/java/org/jenkinsci/plugins/ghprb/GhprbPullRequest.java
	src/main/java/org/jenkinsci/plugins/ghprb/GhprbTrigger.java
Attempt to use the PR URL from the Github API before building our own.