fixed test method being inlined

That would cause ApprovalTests to fail finding the NUnit method invoking the test and fail the test with unhelpful error
added resharper settings file
decoupled conventiontests classes from NUnit

split the whole codebase into distinct testing-framework-agnostic and nunit-specific (as small as possible) parts
also split the types into their own files for better readability/manageability
fixed #3

Removed Fail from Item description and test description