Logical Operators

I found some logical predicate functions useful recently, so I added them to <code>toolz</code> to complement...<code>complement</code>. Does it seem reasonable? The implementation and testing here isn't necessary the final result, just a proof-of-concept.

Also, I wasn't sure whether to call the new functions the imaginary verbs <code>conjunct</code> and <code>disjunct</code> (they're only nouns and adjectives in my dictionary) as per standard functional style or the clearer but longer <code>conjunction</code> and <code>disjunction</code>. Went with the latter for now.