Swift3 updates to Markingbird

[jimstudt]

This will update Markingbird for Swift 3 in Xcode 8.2.1.

There is a lot of simple syntax changing and parameter changing.

More significantly, the default immutability of the 'struct' collided heavily with an escaping closure. There may be another way around it, but I changed the Markingbird object to a 'class' instead since it does heavily mutate itself anyway. This could have an impact on clients if they thought they were getting independent copies by copying Markingbird variables.

There are three failures in MDTestTests related to recognizing back ticks and over quoting an ampersand at one point. I don't know if these are new to the Swift3 port.

[kristopherjohnson]

I will merge this in if someone else will resolve the merge conflicts with the current version of master.