Chaining with use method

[matt-oakes]

A quick PR which adds the missing unit test from #69.

Allows code such as:

backend.use("submit", function(shareRequest, next) {
    console.log("submit");
    next();
  })
  .use("query", function(shareRequest, next) {
    console.log("query");
    next();
  });

[coveralls]

Coverage increased (+0.002%) to 95.654% when pulling d5ce47d on matto1990:chaining into 5dda246 on share:master.

[coveralls]

Coverage increased (+0.002%) to 95.654% when pulling d5ce47d on matto1990:chaining into 5dda246 on share:master.

[coveralls]

Coverage increased (+0.002%) to 95.654% when pulling d5ce47d on matto1990:chaining into 5dda246 on share:master.

[nateps]

Awesome. Thanks!