Add Carthage for watchOS, fix macOS min deployment target version

[dreampiggy]

Dependent pinterest/PINOperation#31
This close #268

CC: @garrettmoon

To make Carthage works (because you use ~> 1.1.0 dependency):
You should firstly merge that PINOperation, release a new version.

Then merge this PR, release a new version.

Note: I also found that Carthage/Checkouts is not in Git Ignore files, does this have anything useful ? Why not ignore those files because when you run carthage update, it will be override and cause massive git diffs.

[dreampiggy]

Local build success (I modify the Cartfile to point to my fork branch):

[garrettmoon]

@dreampiggy Thanks for putting this up! I'm going to merge and put up another PR to update Carthage to use PINOperation 1.2.

With regards to the checkins, I'm not 1000% convinced it's the best course of action, but the intention is to make it so the repo can be built as is without fetching dependencies.

[dreampiggy]

Finally...This get merged :)

[garrettmoon]

Ah! I forgot the real reason we include checkouts is because the Xcode projects depend on them: #280

[dreampiggy]

#280

You can just use add framework dependency using relative path. Then each user just need to run carthage update after cloning your repo, and open the Xcode project to build.

This is standard what package manager should do, like CocoaPods (You need to run pod update after clone the git repo...So your git repo size can under control).

[dreampiggy]

@garrettmoon #279. Please check this to fix current watchOS Carthage issues.