Added ability to query slaves for executors.

  - can pull list of executor objects for
    each slave
  - added unit tests for new code.
Added systests for executor feature. Other fixes.

 - fixed missing index in executor.
 - removed unnecessary try/except block in executors
Randomize jenkins http port for systests to avoid collision with default 8080
Adding myself as a contributor
Added build.get_revision_branch method (Git only)
Made invocation.get_job_number to poll job
Added delay to let Jenkins to catchup with invocation
Added another delay to let Jenkins catchup
Removed unused import
Jobs without the first build can throw error
add unit test for jobs without first build
Changed eval() to ast.literal_eval() for safety
Fix: error message was not giving job name
Updated api module

  Added optional username and password to all methods
  Fixed #203
  Fixed #201
fix for block until queued error on jenkins server with prefix / context path, removed urlparse.urljoin which stripped the prefix
Work on credentials
Added json for adding ssh slave