Update DownstreamContext.groovy

'SUCCESS', 'UNSTABLE' and 'FAILURE' were wrongly mapped to 48, 49 and 50 (ascii values of '0', '1' and '2'). Now the threshold names are mapped to their actual integer value instead of their ascii value.
Fix publisher tests

Fixed test to check against integer instead of ascii values.
Added Github Commit Notifier publisher.

Can be used in the publishers block using 'githubCommitNotifier()'.
Removed plugin version attribute.
Added 'Execute concurrent builds if necessary' option for jobs.
Adding checks for blocking threshold ordinals for the case when downstreamParameterized in step context.
Ensure that block has length zero in publisher context
Merge pull request #162 from eveoh/publisher_github-commit-notifier

Added Github Commit Notifier publisher.
Set default value to true. Added test for default value.
Merge pull request #163 from eveoh/job_concurrentBuild

Added 'Execute concurrent builds if necessary' option for jobs.
Merge pull request #149 from nucl303/patch-1

Parameterized Trigger Plugin support: Fix ordinal of blocking threshold