Updated splash screen to show more relevant information

Theese changes are made to the splash screen
* Branch name was hidden when if name == "master", now it is hidden if
  the current commit is in the history of (origin/)master
* Latest commit datetime is shown on the same line as the version
  string.
* The age of last commit in (origin/)master is calculated at runtime
  startup and shown to the user as # days old master. This is to encurage
  users that compile and follow master to stay current.
* If julia was compiled from a fork the number of commits not in master
  is shown on startup.
* For tagged releases all git info is hidden in the splash screen.
Added a make flag TAGGED_RELEASE_BANNER

As the commit information line is hidden when compilling a taged
release, this adds a make flag so that distributes can cusomize 45
characters of the splash screen.
Fix build when git is not availible
Merge pull request #4665 from ivarne/no_branch_tag

Removed branch info from splash screen on tagged commits