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)

The functionality previously provided by the LoadBuiltInTypes method is now handled by the LibraryServices library loading functionality. The DynamoLoader is only used now to load custom nodes from the definitions folders or types that derive from NodeModel from assemblies in "nodes" folders. It has a convenience method for loading packages which calls the package loader.

We should either go with one loader mechanism, or several discrete loader mechanisms (like the PackageLoader), but having some of this functionality here and some elsewhere is confusing. Regardless of which path we take, we need to ensure that these loaders are using the same assembly resolution mechanism so we can be assured that loading is happening from the prescribed locations. 

This refactoring is outside of the scope of this pull request though :)