watchOS support

[MarcoSero]

This PR is against the protocol extensions branch.

• watchOS target successfully building
• watchOSTests passing all tests

Quick does not seem to support watchOS at the moment.

UPDATE: unfortunately the watchOS test target can't be added until Apple fixes rdar://21760513

[neilpa]

You should be able to use the same shared header as the iOS and Mac targets.

[neilpa]

Some comments around the build setup.

[MarcoSero]

Thanks @neilpa for reviewing .xcodeproj files. I know it's not fun.
I'm planning to tackle Quick/Quick#333 next so that I can add the test target.

[neilpa]

Cool, I know making build changes also isn't fun.

[JaviSoto]

Any updates @MarcoSero? :)

[MarcoSero]

Sorry I was on holiday for the past couple of weeks. I need to find some time to add watchOS support to Quick/Quick#333

[MarcoSero]

Unfortunately the watchOS test target can't be added yet (see rdar://21760513)

@JARinteractive opened the same PR #2206 . We can merge his or mine, I just need to rebase on top of the latest Swift 2 branch.

[MarcoSero]

Squashed commits and opened a new PR targeting swift2 #2209