Ensure JavaScript is evaluated on main thread

[brianmichel]

When using UIWebView we need to make sure that any JavaScript evaluation
is performed on the main thread, otherwise you risk the potential of
WebKit crashing your app in the background.

[brianmichel]

Fixes #516

[E-B-Smith]

@brianmichel Thank you for the patch!

[brianmichel]

@E-B-Smith the tests do not currently run for the sample project, should I add a test for this new code anyways?

[E-B-Smith]

Wow, adding a test would be fantastic.

I know the tests don't compile and run right now. I'm in the middle of refactoring the code and tests, but the tests haven't been updated in way too long obviously. Getting all the tests to run is a high priority.

I was just typing up a blurb about our new beta test project when you sent your PR. You seem like and ideal candidate for the project.

Here's the blurb so far:

We are starting an SDK beta test program at Branch.

We would love to get new features to developers quicker but we can't disrupt the majority of Branch developers who depend on the Branch SDK to simply work.

New features and fixes inevitably break things. The Beta project is for developers who want features and fixes quicker, are aware that some things may break, and are generous enough with their time to do extra testing, report problems, send feedback and make suggestions.

If you want to beta test the Branch SDK, simply use the Github BranchMetrics/ios-branch-deep-linking 'beta' branch:

git checkout beta

If you use Cocoapods, your Branch pod line in your Podfile will look like this:

target "TestBed-Swift" do
  pod 'Branch', :git => 'https://github.com/BranchMetrics/ios-branch-deep-linking.git', :branch => 'beta'
end