Remove the unused and undocumented measurePerformance flag.

To measure the time taken by an activity, there is still
the Time.timed() method available in com.typesafe.dbuild.utils
Further simplifications in DBuildRunner
Print the full stack trace always when debugging
Implementation of custom list of test tasks

This commit adds the new sbt-specific option "test-tasks", which
can be used to customize the testing stage.

It also adds the new "post-commands" and "sbt-post-commands",
which are the counterparts of the existing "commands" and
"sbt-commands". The post-commands are run after building and
testing, and can also be used to run arbitrary shell commands
after testing has completed.
Docs on post-commands and test-tasks
Merge pull request #142 from cunei/wip-vartests

Implementation of custom list of test tasks, and post-commands