Dump library to XML on shortcut

#### Purpose

It is should be possible to dump tree of currently loaded nodes into XML. This PR implements it. For leaves next properties are saved:
* FullCategoryName
* FullName
* Name
* Description.

After pressing `CTRL + ALT + D` the dump is created.

#### Modifications

Added function `DynamoModel.DumpLibraryToXml` which are goes through all nodes of Library tree and serializes them into XML file.

Created new command for shortcut combination.

Example of generated file you can find in [MAGN-5519](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5519) attachments.

#### Unit tests

Added new class `DynamoUtilitiesTests.XmlHelperTests`.
![image](https://cloud.githubusercontent.com/assets/8158551/5187357/2f78349e-74d5-11e4-81a3-ee52a5b9693f.png)

Added test to `Dynamo.Tests.CoreTests` class
![image](https://cloud.githubusercontent.com/assets/8158551/5202377/2e0dfe66-7586-11e4-932b-d3c0bc96d4b5.png)

#### Additional references

[MAGN-5519](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5519).
[MAGN-5520](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5520).

#### Reviewers

@sharadkjaiswal, please, take a look.
you are modifying the state of ViewModel and never rest it at the end of the test. Other tests will get impacted.
Better to add this test in LibraryServicesTest and don't assume that particular library is already loaded. You must load explicit library and validate that all nodes from library is available in xml dump.
You should also add test for cases where a particular node (e.g. hidden nodes) is not expected in the library and hence not present in dump.