Move UIWebView from Twitter (?) to Core/Browser subspec
Differentiate between internal and display handler names

Pretty much every in-app handler is going to be called "In App". This
adds in logic to internally refer to activities by an internal
identifier, which may or may not be the display name specified in the
plist.
Add INKTweetSheet class
Switch templating to use double/triple braces

It's become necessary to be able to escape or not-escape template data
on a per-action basis. This now follows Mustache's lead to have `{{ }}`
represent HTML-escaped data and `{{{ }}}` represent unescaped data.`
Add canPerformAction: check for INKPresentables
Disable forward/back buttons in UIWebView by default