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.
I think it will be good to log flattened list into the library items sorted alphabetically in one section and then the tree hierarchy in another section by referencing these items based on it's id or name if it's unique. This will help us compare the two dump files very easily.