modules/jira: Add some API calls utility functions

Change-Id: a11cfde7c6
Implement release start for JIRA

This change consists of two parts:
  1) Update commands/release/start.go
  2) Implement the common.NextRelease interface for JIRA

The steps should follow (but perhaps in a slightly different order):
  1) Make sure the repository is clean, i.e. there are no files that are
     tracked and modified.
  2) Make sure the remote repository is fetched and develop is up to
     date.
  3) Make sure there is no release branch in the repository.
  4) (JIRA) Fetch JIRA version associated with the version string as
     committed on the trunk branch since this is going to become the
     release version now.
  5) (JIRA) Walk develop to get the additional stories to be assigned
     into the next release. These are the stories affected by commits
     since the last release tag.
  6) Prompt the user to confirm these stories and the whole release.
  7) Create the release branch on top of develop.
  8) Commit NEXT as the version string into develop. In case NEXT is not
     specified, auto-increment the current trunk version string.
  9) (JIRA) Add the issues detected on develop into the relevant JIRA
     version.
 10) Push the modified branches.

Change-Id: e2f5a318d1