Add a "set" directive parameter

[chris]

Add a "set" parameter for directives so that new action variables can be created by evaluating existing action variables.

See this example for a use case:

    SNMP nap1_inodes_vol0_rrd:
       host='nap1'
       oid='1.3.6.1.4.1.789.1.5.4.1.7.1, 1.3.6.1.4.1.789.1.5.4.1.8.1'
       community='public'
       maxretry=10
       rule='not failed'
       scanperiod='5m'
       set='size = %(response1)s+%(response2)s'
       action=elvinrrd("inodes-%(host)s_vol0", "used=%(response1)s","size=%(size)s")
       console=None