Update README with documentation for Cpanel module

- Use atx-style headers
- Add Cpanel module usage docs
- Add "Extending Lumberg::Cpanel" guide
- Update note on sanitizing cassettes
Merge pull request #35 from drichert/enhancement/cpanel-docs

Add docs for Lumberg::Cpanel
Add add_mx and change_mx functions to Lumberg::Cpanel::Email
Merge pull request #36 from mzahir/feature/cpanel-mx

Add CPanel Email MX options - change_mx and add_mx
Add set_mx_type and del_mx to Lumberg::Cpanel::Email
Merge pull request #37 from mzahir/enhancement/cpanel-mx

Add set_mx_type and del_mx to Lumberg::Cpanel::Email
Change the response key from :key to :response_key

Lumberg used :key (within the options hash) to figure
out which parameter should be the key of the response. This
created a conflict with the WHM/cPanel calls that require :key
Merge pull request #38 from mzahir/fix/rename-key

Rename :key to :response_key
Add cPanel SSL support
Merge pull request #39 from mzahir/feature/cpanel-ssl

Add support for cPanel SSL classes