Implement does_not_have_* shortcodes, fixes #51

User can use shortcodes to display messages to users who do not have
specific products or subscriptions:

    [memberful does_not_have_product="1-my-product"]
    You do not have my product :(
    [/memberful]

    [memberful does_not_have_subscription="2-my-sub"]
    You do not have my subscription, please buy it!
    [/memberful]
Fix does_not_have_sub and does_not_have_product being used together

Not entirely convinced this should be used given discussion about
`else`, but committing for posterity
Coding standards changes, refs #51
Merge pull request #51 from memberful/51-custom-protect-message

Custom message for protected content