Enabled firebase to be called as function, thus enabling patterns where Firebase ref's are stored in root component (#25)

* Enabled firebase to be called as function

* Added basic test case for ability to run as function

* Deleted compiled files

* Revert "Deleted compiled files"

This reverts commit 327203a.

* Revert "Revert "Deleted compiled files""

This reverts commit a11bedd.

* Revert "Deleted compiled files"

This reverts commit 327203a.

* Changes to test spec

* Updated test to be more simple

* Removed regex from throw, added Sinon spy to call

* Changed line order of check

* Moved the correct line :P

* Reverted changes to dist files

Author: DaBs

src/vuefire.js

@@ -213,6 +213,7 @@ function ensureRefs (vm) {
 
 var init = function () {
   var bindings = this.$options.firebase
+  if (typeof bindings === 'function') bindings = bindings.call(this)
   if (!bindings) return
   ensureRefs(this)
   for (var key in bindings) {

tests/vuefire.spec.js

@@ -25,6 +25,19 @@ describe('VueFire', function () {
     })
   })
 
+  describe('support Function options', function () {
+    it('option is callable as function', function (done) {
+      var spy = sinon.spy()
+      expect(function () {
+        new Vue({
+          firebase: spy
+        }).$mount()
+      }).to.not.throw()
+      expect(spy.calledOnce).to.be.true
+      done()
+    })
+  })
+
   describe('bind as Array', function () {
     it('throws error for invalid firebase ref', function () {
       helpers.invalidFirebaseRefs.forEach(function (ref) {