Packages Directory

This pull request replaces the package manager's current logic for finding the "dynamo_packages" path with the use of the DynamoPaths.Pacakages property. The DynamoPaths.Packages property is set at run-time to:

<User>\AppData\Roaming\Dynamo\0.7\packages

All reading of packages and downloading packages should happen in this directory. This fixes a bug present in Dynamo since moving the install to C:\Program Files that the packages directory could not be created due to the user's lack of permission to write into folders inside program files. The appdata folder is the recommended location for application-specific files per-user. And installing these files in roaming allows the files to travel with the user when they log into different machines around the domain.

This is for @pboyer to review as he implemented the original package manager path-finding.