Added tests for ModuleInstaller.

Test that a mod can be installed in a clean KSP instance.
Test that a mod can be removed after installation.
Test that ModuleInstaller instances are decoupled when working on multiple KSP instances.
Decoupled ModuleInstaller instances based on KSP instance GameDir.
Merge pull request #31 from mgsdk/refactor_singleton_moduleinstaller

Refactor ModuleInstaller.GetInstance() singleton, decouple instances