Implemented the new Start Page for Dynamo

Background
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This pull request implements the new ```Dynamo Start Page``` (internally tracked with [this defect](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3754)) as described in [the specification document](https://docs.google.com/a/adsk-oss.com/document/d/1o8xNWbpdd6d4F-3HkNtu_uaC9aE69zELMAD0bGs1SLM/edit). The Start Page aims to provide a seamless onboarding experience for many first time users by giving them one-click access to various online resources.

The Start Page provides easy access to the following online resources:

- [Discussion forum](http://dynamobim.org/forums/forum/dyn/)
- [Email Dynamo team](mailto:team@dynamobim.org)
- [Dynamo official site](http://dynamobim.org/)
- [PDF tutorials](https://github.com/ikeough/Dynamo/blob/0.6.2/doc/distrib/Dynamo%20Visual%20Programming%20for%20Design.pdf?raw=true)
- [Video tutorials](http://dynamobim.org/learn/)
- [Dynamo GitHub repository](https://github.com/DynamoDS/Dynamo)
- [Issue reporting](https://github.com/DynamoDS/Dynamo/issues)

In addition to online resources, the following can be achieved through the Start Page:

- Create a new document or open an existing one
- Access to recent file list
- Access to sample file list

@kronz and @mccrone, check this out ;)

![dynamo-start-page](https://cloud.githubusercontent.com/assets/5086849/3422628/01d967c6-ff53-11e3-8276-0fed3e25d239.png)

Testing Scenarios
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- ```Start Page``` should show up when Dynamo launches
- ```Start Page``` should go away when creating or opening a document
- Pressing ```X``` on the ```Home``` workspace tab should bring back ```Start Page```
- Pressing ```X``` on the ```Home``` workspace tab does not show ```Start Page``` if at least one ```Custom``` workspace is open
- Clicking on all buttons should bring to respective resources
- Clicking on ```New``` or ```Open``` should have the same outcome as clicking on the tool buttons
- Opening a ```DYF```/```DYN``` file will result in ```Recent``` list being updated

Notes to Reviewers
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Hi @lukechurch, guess you're still providing weekend review services, if you get to this, good, otherwise I can always wait for @ikeough to come back later. Thanks :)

The changes are adequately commented hopefully that helps in your review process. I have noticed a few issues with existing code base but in the interest of keeping changes to minimal, I have created [a new task](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3813) to track those internally.
The above statements about incurring the cost of construction are true, if you just insert that start page as an item in the main view's grid. But, if you think of the start page like the way we do workspaces, you can get around this. For workspaces, WorkspaceViewModel objects are inserted into the source collection for an items control. Construction of the view doesn' t happen if a view model is never put in the collection. Why don't we just add an items control, the item template for which would be a start page iew. Then when we want the start page, we insert a start page view model in the collection, and when we want it to go away, we remove it.