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 in quick comparison of the two dump files easily.

I have created task for it [MAGN-5542](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5542). I would prefer to create additional PR for easier review process.

> Each element data should have enough information, so that nodes can be created from element definition directly without loading all the libraries. We may also need information about dll and possible node type.

Node type is specified in tag name. What about another needed information I would like to add it later in separate PR when I will do all research to understand how loading process done, what is needed for every node type. It can be not simple adding one or two properties. For `DSFunctionNodeSearchElement`, we have, for example, `Descriptor` which should be passed to constructor of it. Have created tasks: [MAGN-5543](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5543), [MAGN-5544](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5544), [MAGN-5545](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5545).

This PR to consider the general view of XML which should be created. Specifics should be separated in their own PRs. 

@sharadkjaiswal, does this make sense?