On MSVC if unicorn isn't found, fallback to bundled unicorn
On everything else, fallback to building unicorn in externals
Also fixes loading unicorn in msvc
Adds a cmake custom target that will build unicorn on first compile and
uses this in the build scripts as well. Updates Appveyor and Travis
build scripts to work with the new unicorn build, and updates the paths
to all of the different artifacts.
