Warn about bad pre/postconditions

[jafingerhut]

See Clojure ticket http://dev.clojure.org/jira/browse/CLJ-1473 for some examples.

[jafingerhut]

Combining notes from nearly-duplicate issue #80 here, and closing #80:

See examples at clojure-dev messages linked below:
https://groups.google.com/d/msg/clojure-dev/i7pijgFaa0I/OoW0V551VWcJ
https://groups.google.com/d/msg/clojure-dev/i7pijgFaa0I/o15ozkWWJq8J

[jafingerhut]

From hiredman on #clojure IRC channel: would like to see a warning for code like this
(defn f [...] ...) (defn g [...] {:pre [f]} ...)

[jafingerhut]

Got the new :wrong-pre-post linter into pretty good shape with this commit: 8b2b702