closes #344 - ability to specify livereload port number

In some test environments (i.e. nitrous.io) you have limited control over which ports are open.  You need to override the location of the livereload port in these cases.

This change adds a new option  --live-reload-port nnnn  to the server command.  The default remains port + 31529.

Added appropriate unit test, and tested solution under fire on nitrous.io

As a follow up it would be good to add some documentation (i.e. type server --help)  to the ember cli web page 
just following instructions https://github.com/stefanpenner/ember-cli/blob/master/CONTRIBUTING.md