[clojure mode] Check if +/- is immediately followed by digit before marking as leading sign

Closes #782

Correct highlighting for the following code snippet should highlight the + as an operator, and each 2 as a number (taken from http://clojuredocs.org/clojure_core/1.2.0/clojure.test/deftest)

    (is (= 4 (+ 2 2)))