Perform URL encoding more lazily

This sets the stage for loading view controllers in INKActivity objects,
where we won't want parameters to be escaped.
Can send email using modal view controller
Create INKPresentable protocol
Make INKActivity aware of its presenting view controller

Necessary to present modal view controllers in the correct context
Backfill tests for view controller / mail sheet behavior
Add CocoaDocs for INKMailSheet and associated code
Fix mocking issues causing test failures
Don't escape commas in URLs
Add in-app modal UIWebView as a web handler
Make in-app handlers the default for mail/browsers
Add toolbar to in-app web browser
Don't show first-party apps if an equivalent in-app version exists

e.g. Safari vs UIWebView, Mail.app vs MFMailComposeViewController
UIWebView share sheet shows in a popover on iPad
Fix failing specs for browser handler
Run tests using xcpretty
Add shorthand INKPresenter `presentModally` method

This attempts to find the front-most view controller, saving you the
trouble of manually specifying a view controller.

Finishes #40
Remove spurious demo code
Hide default toggle when it's not settable
Handlers can explicitly disallow the 'Remember my choice' toggle
Move  helper method into IntentKit class