Extracted CecilExtensions.CalledMethod to a standalone class, MethodCall

Use it as return value from TestCase.GetCalledAssertingMethods and also in the call graph.
Mini cleanup
Extracted AssemblyLoader from TestCases so that we can use it in TestHelper and thus get debug information when creating a test case from an anonymous function.
Added AvoidAssertBooleanRule

This rule checks that a test case doesn't perform a boolean assertion of
a composite boolean value, e.g. checks that x == y is true.