Save-upload functionality for NWK

It should be implemented in the different from browser version way. Flood in NWK should have the same kind of behavior that is in DynamoSandbox:
1) Open - Open a file from the local file system. Dynamo will need to know about the location on disk in order to support 2. 
2) Save - Save the current workspace to the local file system - this is only relevant in node-webkit mode. If the file already has a filename associated with the local file system, overwrite that file.
3) Save As.. - Prompt the user to provide a new filename to be saved to the local file system.

This PR contains changes from https://github.com/Benglin/Dynamo/pull/152. So https://github.com/Benglin/Dynamo/pull/152 should be merged first
Try this for more informative exception:

```csharp
throw new Exception(string.Format("Failed to save file: {0}", filePath));
```