Add targets to enable Atlassian Clover code coverage.

Clover is available for opensource projects from:
http://www.atlassian.com/software/views/open-source-license-request

1) Add Clover code coverage targets clover targets are guarded from being called without clover.jar being passed
In order to execute clover code coverage you must pass -Dclover.jar=<path to clover.jar> to the ant call along with calling the with.clover target. The following example will compile mesh with code coverage, execute the test suite then produce a code coverage report

  $ ant -Dclover.jar=clover.jar with.clover test clover.html

2) Add the ability to pass jvmargs to the test ant target via -Dtest.extra_margs

I have been collecting coverage results using the following runs:
  $> ant -Dclover.jar=../clover.jar with.clover test
  $> ant -Dclover.jar=../clover.jar -Dtest.extra_vmargs=-Dcompile.analyze.ConstantReducer.ENABLED=false with.clover test
  $> ant -Dclover.jar=../clover.jar -Dtest.extra_vmargs=-Dcompile.gen.java.inline.TermInliner.ENABLED=false with.clover test
  $> ant -Dclover.jar=../clover.jar clover.html

You can also run the instrumented shell with the following:
  $> CP_APPEND=../clover.jar ./bin/shell