aligned composer.json with other packages in ZF2

require php 5.3.3, set a branch alias for 2.0.x, do not require phpunit
Cleanup.

- Fix imports.
- Fix CS.
- Fix docblocks.
- Add argument validation for CpuPerformance::__construct()
Fix CS.
Add missing docblocks.
Add Check\DiskFree with tests and README.
give some margin of error in the diskfree test
Fix CS.
Merge remote-tracking branch 'origin/feature/diskfree' into feature/diskfree