Several Updates

- Google Wallet integration cleanup. See: WebSharks/s2Member#19
- Specific Post/Page Access now possible up to 50 years. See: WebSharks/s2Member#24
- Correcting references to "Add a Member". See: WebSharks/s2Member#23
- Documenting upper limit on Membership Levels. See: WebSharks/s2Member#22
- Documenting upper limit on Membership Levels. See: WebSharks/s2Member#22