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

![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

This class represents the code-behind of Start Page. It handles the following tasks:

- Populate all the list boxes on Start Page
- Handle "Recent" file list notificaitons and update the list
- Handle file item clicks to open the corresponding files
- Handle command item clicks to invoke respective commands
- Handle external link item clicks to navigate to the URL