Infer AssemblyFile if TaskFactory is used

It's no small annoyance to have to know the full file name as well as versioning scheme
of the tasks assembly, which has been a moving target in almost every version of MSBuild.

To the point that I ended up creating a nuget package that gives me a `$(CodeTaskAssembly)`
that [works reliablely across all MSBuild versions](https://github.com/MobileEssentials/MSBuilder/blob/master/src/CodeTaskAssembly/build/MSBuilder.CodeTaskAssembly.props#L35-L43).

So, how about just having MSBuild automatically fill the `AssemblyFile` for me whenever I
use a `TaskFactory="CodeTaskFactory"` attribute in a `UsingTask`? Or maybe even more concise
yet, just give me a separate element name for it, like `UsingCodeTask` or something and do away
with the attributes altogether :)

May be a superset of what's asked for in #897?