MXS-1300: Un-modularize the core

Making the core a module proved to bring more problems than it solved. For
the sake of simplicity in installation and code coverage reporting, the
core is now completely contained in the `lib/` directory.

Also added preliminary coverage reporting with nyc.

maxctrl/test/states.js

@@ -1,5 +1,8 @@
 require('../test_utils.js')()
 
+var ctrl = require('../lib/core.js')
+var opts = { extra_args: [ '--quiet'] }
+
 describe("Server states", function() {
     before(function() {
         return startMaxScale()
@@ -8,9 +11,6 @@ describe("Server states", function() {
             })
     })
 
-    var ctrl = require('maxctrl-core')
-    var opts = { extra_args: [ '--quiet'] }
-
     it('set correct state', function() {
         return ctrl.execute('set server server2 master'.split(' '), opts)
             .then(function() {