Update parallel processing api (#246)

* Remove m.workers, add run(procs=), update docs

* Update docs

examples/linux/binaries/concrete_solve.py

@@ -29,7 +29,7 @@ def solve(state):
 
 # play with these numbers!
 m.verbosity = 0
-m.workers = 1
+procs = 1
 
-m.run()
+m.run(procs)
 print m.context['solution']

examples/linux/binaries/symbolic_solve.py

@@ -33,6 +33,6 @@ def solve(state):
 
 # play with these numbers!
 m.verbosity = 0
-m.workers = 1
+procs = 1
 
-m.run()
+m.run(procs)