Feature/add nuget package (2/2)

Here's the pull request. I'm actually pretty satisfied. Just:

- install the FSharp.Tools nuget package.
- Restart Visual Studio
- In the build actions, now you see _FsharpLex_ and _FsharpYacc_, no more fiddling with project files by hand :D

In case of errors, the std error message show up as compile errors (I changed printf for eprintf for better error logging).

It even notices file changes in the fsl files and only re-fsharplex/fsharpyaccs it then.


It is based off the other one.