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)

These calls can fail for a very wide variety of reasons (access deny, bad image format etc). I think that the code needs to be explicit about its error handling behaviour, and whether it is expected to handle the errors or its caller is.