Allows the last statement to end in semicolons.

Previously, if you had something like:

    $do {
      a <- b;
      return c;
    }

The macro would compile it down roughly to:

    a.chain(function(b) {
      return a.of(c).chain(function(_it) {
      })
    })
Fixes variable binding example.
Fixes macroclass syntax for 0.7 release.

`macroclass` syntax was changed slightly shortly after the 0.6.0
release. See: https://github.com/mozilla/sweet.js/pull/350
Provides tests for semicolon support.