Cleans the framework before building and allows customization of some params through env vars.

This modifies the generated ruby script to delete the target framework directory before building it. Otherwise, you can end up with old files in there if you remove files in a newer version.

This also makes several variables customizable through environment variables that can be set in the post build script. My use case didn't use the library name as the name of the target and had a slightly different name for the static library. Also, the default header directory set by xcode 5.1 was /usr/local/include rather than the script default of ${PROJECT_NAME}Headers.

If the environment variables are not specified, the values default to how the existing script works.