Assemblies loading for Policy Evaluator and Checkintool improved. Refs #258
Move the path resolver to the constructor.

This will ensure that it's in place before evaluating
checkin policies, which is when it is needed, without
expanding the interface of ITfsHelper.
Check to install resolver only once
Merge pull request #573 from KindDragon/checkintool

Assemblies loading for Policy Evaluator and Checkintool improved