Added test-cov command
Merged TestCovCommand into TestCommand, but with switch
Merge pull request #385 from ColdenCullen/master

Added test --coverage switch.
Remove the build type switch for "dub test". See #385.
faster determinePlatform

- 310ms -> 7ms

- don't link dub_platform_probe

- avoid phobos imports in the probe file
store metadata cache on disk

- Writing is a little slow due to Json escaping
  so also use a dirty flag to track when a writeback
  is needed.

- The metadata contains readme files for every project,
  strip that data for now because of the point above.
create symlinks instead of copying cached build files