Separated UnitAttribute out into separate file and moved both into Attributes folder.
Added __Regenerate.tt to generate all implementations from T4 templates.

* Converted UnitClasses.tt and UnitConverter.tt to .ttinclude files and moved into Includes folder.
* Converted UnitsNet.net35.csproj to not using wildcard include anymore, it was very inconvenient when developing. The other 3 projects should still use wildcards as they are not developed on.
* Modified MultipleOutputHelper.ttinclude to add output as file in same folder as template file instead of include as nested item under template file, to better match solution structure in other 3 projects.

Issues:
* <#@ include once="true" #> does not seem to work and I get duplicate definition build errors. Commented out includes in .ttinclude files and losing intellisense in those files.