Add support for listening to long-clicks

[saket]

Fixes issue #5.

@ehehhh @debjitk, would you like to take a look at this? The logic for registering long-clicks was taken from View.class.

You might also want to read the final class instead of going through the diff, because I ended up moving things around for making the code more understandable.

[saket]

@madhu314 I also made another change, where ClickableSpanWithText is only created when a click/long-click callback has to be dispatched instead of on every touch event.

This way,

• a callback is only made if the click started and ended on the same URL
• unnecessary allocations are avoided

Since this was originally written by you, I hope this doesn't break anything.

[ehehhh]

Looks good, man! 👍