Added TestResultLogger for customising test feedback.

1) When `test` is run and there are no tests available, omit logging output.
   Especially useful for aggregate modules. `test-only` et al unaffected. (#1185)

2) Added a new setting `testResultLogger` to allow customisation of logging of test results.
Fix infinite loop of death.
Merge pull request #1245 from sbt/fix/infinite-loop

Fix infinite loop of death.
Merge pull request #1225 from japgolly/shhh_tests

Added setting 'testResultLogger' which allows customisation of test reporting.
Upgrade sbt plugins and version with sbt-git.

* Version number now includes git SHA for snapshots, or the tag.
* Version base string now encoded in `version.sbt`
* Upgrade to latest sbt-site, sbt-git, sbt-ghpages.