Updated definition of plugin priority metadata field.
Added plugin sorting function that puts masters before non-masterrs.
Issue #39 work. Started to implement graph sort.

Also moved graph-related stuff into a new h/cpp file pair, and fixed the
boss::master_sort function. For some unknown reason, an exception gets
thrown after roughly 180 operations on the graph object, ie. near the
end of the out-edges addition for the first plugin in the list.