Added the ability to always save session data.

Added an extra <Manager../> attribute sessionPersistPolicy which
can be either DEFAULT or ALWAYS. If the chosen value is ALWAYS,
the session will always be persisted at the end of serving the
request, regardless of whether setAttribute() was called with a
different value for an existing attribute or not.

The sessionPersistPolicy is optional, defaulting to DEFAULT
Added maven project file, increased version to 1.3-SNAPSHOT and ignored IntelliJ Idea project files.