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.
Added Headless property to IUser
Small fix
Merge pull request #33 from AlexanderDzhoganov/master

Added Headless property to IUser
Tests for RelationshipResolver with version support.
Small fix to without_enforce_consistency
Merge pull request #35 from AlexanderDzhoganov/master

Small fix to without_enforce_consistency
Added enforceConsistency flag to RegistryManager.Save
Merge pull request #36 from AlexanderDzhoganov/master

Added enforceConsistency flag to RegistryManager.Save
Merge pull request #34 from Ormira/versions_#17

Tests for RelationshipResolver with version support.
Merge pull request #31 from mgsdk/refactor_singleton_moduleinstaller

Refactor ModuleInstaller.GetInstance() singleton, decouple instances