New dm4-time plugin: store timestamps (#386).

"DeepaMehta 4 Time" is a new plugin.
Belongs to the DeepaMehta Standard Distribution.

For every topic and association 2 database properties are maintained:
    - "created"
    - "modified"

For readability timestamps are stored in IETF-compliant RFC 2822 format with a fixed GMT/UTC timezone.
Example:
    Tue, 04 Jun 2013 00:23:16 GMT
See http://tools.ietf.org/html/rfc2822#section-3.3

Note: you can investigate the timestamps via the Neo4j Shell.
Example:

    mvn exec:java -P neo4j

    neo4j-sh[readonly] (0)$ ls 2695

        *node_type =[topic]
        *uri       =[]
        *type_uri  =[dm4.notes.note]
        *value     =[T1]

        *created   =[Tue, 04 Jun 2013 00:23:16 GMT]
        *modified  =[Tue, 04 Jun 2013 00:23:40 GMT]

        *creator   =[admin]
        *owner     =[admin]
        *acl       =[{"WRITE":["CREATOR","OWNER","MEMBER"]}]

See ticket 386.