Bump Sammy.js to 0.7.4

0.7.2 is no longer available for build.
cleaned up extra blanks at eol
sammy was upgraded, correct test
socket.io.min.js is only available for pre-1.0
Added X-Powered-By: Zappa header (#68)
Tests for `X-Powered-By` (#68) and `format` (#63)
Added example to show `format` with explicit mime-type. (#63)