Add uninstall target to build system

By default cmake doesn't actually provide an uninstall target for make like other build systems, it expects the package manager to handle this for it. The issue is right now there's a lot of distributions which don't have any sort of package for installing corebird, and may very well require the user install/uninstall it by hand. The cmake wiki provides a solution for this, so I've modified it to meet the indent style of the other files.
The only drawback to this solution is that `make uninstall` won't work unless `make install` is ran at least once, since it requires the install_manifest.txt file that cmake creates during installation in order to work. The second commit clarifies that in the error message that's printed in the event of a missing install_manifest.txt so as to make it less confusing for the user to fix. It's really not the best solution, and if I get the time I'll probably try to write a better one for cmake, but IMO it's better then nothing.