Added waiter actions to resources

For each resource that has a resource waiter definition a ``wait_until_...``
method was added to the resource. Documentation was updated as well to
document both the resource waiter and client waiters.
Add a section for client waiters
Touched up the waiter code
Merge pull request #43 from kyleknap/waiters

Implement Resource Waiter Actions
Added more docs about waiters

Added docs to both overview of clients and resources.
Merge pull request #44 from kyleknap/waiter-top-level-docs

Added more docs about waiters
Update SQS resource model
Update changelog
Bump version to 0.0.6