v2.2.0: add an about section

Added an about section to the README to help explain the purpose of
this package.

v2.1.0: add travis configuration

Added Travis CI configuration and CodeClimate reports.

v2.0.0: remove counter

Removed the need for an atomic counter for the commitId by combining
the committedAt and aggregateId properties. This assumes that a single
aggregate will not have more than one commit per millisecond. In practice
this should hold true as a version conflict would likely occur. In
addition, callbacks were removed and promises used instead. The AWS
object has been exposed to allow for custom Promise implementations.

v1.0.0: append, query, scan

Added basic functionality.