Build on *nix mono enviroment

Hi,

Currently we could not be able to build katana on Mono environment because of post/prebuild command.
I've made changes on .csproj files to let *nix to build katana.

However Mentalis security library relies on crypt32.dll which mono doesn't have, so we can't run anyway...

Please accept my pull request if it's still worth merging.

Thanks,
Hiro