Revit Sub Folders

This pull request reorganizes the outputs of a build such that everything with a Revit dependency is moved into a revit subfolder of the build directory corresponding to a revit release. For example, all Revit 2014 libraries can now be found in bim\AnyCPU\Debug\Revit_2014. This sub folder contains its own "nodes" folder which is searched for UI nodes types.

The assembly resolution logic has been modified to search first in the "core" directory and then in the "dll" directory. A new class, DynamoPaths, is added to hold the directory paths for key asset loading paths.

![revitsubfolders](https://cloud.githubusercontent.com/assets/1139788/3224433/e0b0df52-f034-11e3-8724-06b917fbe3f7.PNG)

We only use this method as a handler to the ApplicationDomain's AssemblyResolve event. As such, catching a failure in this method and returning null could leave Dynamo in a bad state (if it works at all). The new try/catch logic which I've added here doesn't change the "fail hard and fast" that we were doing before, with the unhandled exception being caught by our crash processing logic, but it makes a slightly more readable message, and makes it more obvious what is going on to those who look inside the method.