Choose custom root SDK path depending on an Xcode location

Set custom root SDK path (using `--sdk-ios` & `--sdk-mac` command line args) to be based on current Xcode location.

The behaviour is from `xcode-select` utility: if there is an `/Applications/Xcode.app`, we choose it. If not — we check for `/Developer`.
Update usage info according to a new `--sdk-ios`/`--sdk-mac` behaviour
Merge pull request #38 from rodionovd/patch-1

Choose custom root SDK path depending on an Xcode location