Cleanup, refactor…

This PR includes the following changes:

* Added rspec tests;
* Added to Travis CI;
* Refactor classes to simplify dependencies;
* Get rid of unnecessary inheritance (use the `params` class for everything);
* Get rid of unscoped variables;
* Rename classes with a dash.

:warning: The following changes break the compatibility:

* The `$dhcp_*` unscoped variables are not used anymore, they are replaced with parameters in classes;
* `$global_options` in `dhcp::hosts` is now an array instead of a string;
* The `'option'` entry in the hash passed to `dhcp::hosts` is renamed to `options` and is an array;
* The `dhcp::shared-network` class was renamed as `dhcp::shared_network`.