Adds a way to set non-boolean build settings.

[StephanPartzsch]

Hi! As discussed on Twitter, I am here now with the pull request. I added a way to set non-boolean build settings with Crafter. Although the way via 'options' is somehow redundant, I kept it because Crafter is a library and I don't want to force any user to change their configuration files. Here is an example how to use it in the config: set_build_settings ({ :'OTHER_CFLAGS' => '-Wall' }) I added a few checks too. In case someone wan't use 'options' or 'build_settings' in the config file. Hope that is all ok with you.

Best, Stephan

[StephanPartzsch]

Oh, I forgot: I have not changed the gemspec. So versioning and stuff is for you ;)

[krzysztofzablocki]

Thank you ;)

[StephanPartzsch]

you're welcome ;)