more robust checking, silent tools install 10.7+

Researched all probable receipts to iterate over which reflect either
CLI tools install or full Xcode on Mavs(which includes the tools.) Made
@timsutton’s script a function to handle conditional silent install
based on OS
Merge pull request #6 from arubdesu/CLITools-receipt-checking-and-silent-install

more robust checking, silent tools install 10.7+