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.
Should be named as LibrarySnapshot$GUID.xml so that it's easy to locate.