Adding Support for SwiftPM

[vknabel]

This PR adds support for macOS-only SwiftPM support.
Currently building is only supported by running xcrun -sdk macosx swift build.
This implementation currently uses some forks instead of the real projects in order to add (currently) missing version tags.

Currently missing:

• Support for swift build
• README.md updates
• CircleCI and TravisCI updates
• Support for swift test

[krzysztofzablocki]

thanks for contributing!
What do you think would be the best approach to follow up on those checkbox items?

btw. swift build works for me on Xcode 8.2

[vknabel]

I will open some follow up PRs. This way others can start to experiment with Homebrew support.

Did you use Xcode's Swift? Mine has been installed with swiftenv.

[vknabel]

@krzysztofzablocki Adding support for swift test would take a bit more time as there are multiple compile errors for the test targets. Shall we merge this PR? I would open another PR later.

Once it's merge-time: in the CHANGELOG.md I used Future Release as a placeholder. Before you merge it I may change it to the next version.

[krzysztofzablocki]

Sounds good to me, you can just put # Master as release version, we gather changes that are in master and then when I do next release I change it to future version :)

Any chance we could change the product name in SPM version to be lowercase sourcery rather than uppercase?

[krzysztofzablocki]

We should update the readme entry to lowercase as well, thank you so much for your help with this :)

[krzysztofzablocki]

@vknabel I think I pressed the merge too fast instead of waiting on CI, any idea why error: these referenced modules could not be found: sourcery would happen ?

[vknabel]

@krzysztofzablocki Git has sometimes problems when renaming files from Uppercase to Lowercase. Opened #64