Don't try to start Crashlytics if the API key was not loaded.
Don't try to run Crashlytics if the API key does not exist (i.e. if key file is missing or empty).
Remove the project dependency on the Crashlytics.apikey file, and instead copy it into the bundle manually if it exists. This way compilation succeeds even if the file doesn't exist.
Removing unnecessary absolute framework search paths that were causing warnings.
Merge pull request #256 from airdrummingfool/fix-crashlytics

Make Crashlytics optional (fixes #250)