[FIX] Search system CA's if the default cacert doesn't include a valid CA [#146 state:fixed]

- By default we only ship the certificate for sks-keyservers
- If the default cacert is not overwritten with a custom one, the system CA (keychain lookup) is checked for a valid CA.
- Adds the functionality to lookup the system CA via gnutls directly to curl.
[FIX] Correctly replaces all absolute paths in dylibs and binaries with @rpath directives [#147 state:fixed]

- Extends the "fix install name" method to also walk through all dependencies of a dylib and change
  the references to @rpath if they currently use an absolute path for locating the dependency.
  Checks i386 and x86_64 dependencies.
- Removes specific install name changes from Formulas, since they are no longer necessary.