Add children command

still some issues to be worked out...
add preliminary lhs/rhs
children(x) should be [x] for SMT compat
more tests for lhs/rhs
children: order out output might depend a bit on sympy version

phase of moon, etc...  these tests might be a bit fragile.  Tweaked
for now.
add setdiff and setxor commands
children: fix unpredicable tests: use set operations

specifically, isempty(setxor()) to tell if two sets are the same.
setdiff/setxor tests more robust to ordering of lists

ironic, b/c I implemented setxor to... give robust setting of
equivalent lists in the set sense!
setdiff test: errata from last commit
improve lhs/rhs

give errors if sympy object has no .lhs or .rhs attribute
documentation: add lhs/rhs to thing we have, SMT doesn't
children: improve documentation

Fixes #139.
Merge remote-tracking branch 'origin/master' into children

Conflicts:
	NEWS
Merge pull request #146 from cbm755/children

add "children" and lhs/rhs commands