fix issues to support case sensitive file systems

I had to put a condition for the AfterBuild task which copies the .pdb so it only do so if the file exists. I added a similar Copy task for .mdb which are the debugging databases emitted by mono's compiler.

In the Fake build script, I had to override the working dir to match with the actual directory containing the .nuspec files.

I'll consider fixing Fake's NuGetHelper but I'm unsure yet what how this folder is supposed to be named.