Various migration manager fixes

This commit addresses a few issues with migration manager mishandling DYN files and missing nodes:

1. Files of version ```0.7.0.xxxx``` no longer get migrated in ```Dynamo 0.7.0.0```
2. Now backup files will be created under a sub-directory ```backup```
3. Unresolved node types now result in ```dummy nodes``` with appropriate information

It fixes the following two defects tracked internally:

[MAGN-1961 backup files keep adding up on every open](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-1961)
[MAGN-1959 Unidentified nodes should be turned into dummy nodes](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-1959)

Various NUnit test cases are added for newer functionality (e.g. backup folder retrieval mechanism)