Only build framework schemes

Fixes #124, #143. Tested with the unit test fixture and also the `Cartfile` described in the original issue.

/cc @0xced
This enum will never be exhaustive, simply because `PRODUCT_TYPE` can be set to absolutely anything. Even `com.apple.product-type.framework.static` specifically is completely arbitrary and _not defined by Apple_ from anything that I can see.

If we add support for building static frameworks in the future, we'll certainly add to the set of possible enum values, but it seems unnecessary and fragile right now.