Expand support for job constraints

 -- node_features plugin - Add suport for XOR & XAND of job constraints (node
    feature specifications).
 -- Add support for parenthesis in a job's constraint specification to group
    like options together. For example
    --constraint="[(knl&snc4&flat)*4&haswell*1]" might be used to specify that
    four nodes with the features "knl", "snc4" and "flat" plus one node with
    the feature "haswell" are required.