Fixes issues #126 and #125: Config options support for sessions

It's now possible to pass config option to the session factory. These options
will be used whenever a Dancer::Core::Session object is created.

Example:

session: YAML
engines:
  session:
    YAML:
      session_config:
        cookie_name: 'session.foo.dancer'
        expires: 86400

All entries in "session_config" will be passed to the constructor.
reflect Changes
example in contrib app
Merge branch 'topic/sessions-config-options'