(GH-44) Add 'options' parameter to node definition

- Add optional config 'options' to RSpecSystem::Node
- Add custom options to Kwalify schema
- Add test fixtures to ensure schema validates
- Add option description to README
(GH-44) Support custom options for Vagrantfiles
Merge pull request #47 from justenwalker/custom-node-options

Add custom options to node definition
Fix wrong reference in README.

When talking about integration with Puppet, I suppose the link is expected to
refer/link to `rspec-system-puppet`.
Merge pull request #50 from stefanozanella/fix_readme_link

Fix wrong reference in README.
Fix typo in example test code.

Looks like `/etc/resolv.conf` has nothing to do with the test suite in the
example.
Merge pull request #51 from stefanozanella/fix_readme_example

Fix typo in example test code.
Add 'ip' option to the 'options' parameter for node definitions
Add a simple plugin loading system for node_set plugins
Merge pull request #53 from treydock/ip-node-option

Add 'ip' option to the 'options' parameter for node definitions
Merge pull request #58 from dalen/master

Add a simple plugin loading system for node_set plugins