add associative unification
rename assoc_unify to unify_assoccomm

add unify_assoc and unify_comm convenience functions
add commutative tests
add failing test for deep associativity
add inline todo
correct spacing between \d,\d
add eq_assoc and eq_comm goal constructors
add docsrings for eq_assoc/comm
add test for op inequality
rename uop to op
Merge pull request #8 from mrocklin/assoc-comm

Assoc comm