Add .gitattributes

C# and whitespace rules.
Ignore build folder and Thumbs.db.
Add initial template.

Example project showing how to add custom units to Units.NET, in order to use UnitSystem class to parse/get abbreviations from units as well as reuse T4 templates to generate unit classes from unit enums with special attributes.

* Copied from https://github.com/InitialForce/UnitsNet
* Foo and Bar units generated with templates
* Tests for unit classes generated with templates
* Tests for UnitSystem with external units
* NuGet packaging prepared
Fix nuget packaging.

* Update spec and fix errors in it.
* Rename project to UnitsNet.ThirdParty.pcl.
* Add project UnitsNet.ThirdParty.net35.
* Updated build and packaging scripts.
Rename output of UnitClasses.tt to .txt.

To avoid compile errors on bad T4 template runs.