Support Swift Package Manager

[DanielKrofchick]

The package can be added to a project but it fails when building, giving multiple Property has a previous declaration errors. The errors occur for properties such as displayInfo and deviceVersion where duplicates are showing in GBDeviceInfo_iOS.h and GBDeviceInfo_OSX.h.

[austinlouden]

I'm able to install via SPM as well, but see the issue:

target 'GBDeviceInfo' has invalid header layout: umbrella header found at .../SourcePackages/checkouts/GBDeviceInfo/GBDeviceInfo/GBDeviceInfo.h', but directories exist next to it: .../SourcePackages/checkouts/GBDeviceInfo/GBDeviceInfo/Supporting Files; consider removing them

[cpsauer]

(I think probably this repo is in more of a self-serve mode. Any chance one of you guys would be willing to toss up a PR? Would do, but I'm currently using via bazel instead of SPM.)