Add IThrowable with -get-message and have Exception implement it.

Add -get-message to Exception to make it possible to print exception
messages (similar to Clojure/JVM's .getMessage).
Escape backslash in string-quote.
Add cljc.string and initial set of string functions to core.cljc.

Add initial implementations of reverse, escape, lower-case,
upper-case, capitalize, blank?, triml, trimr, trim, and trim-newline,
along with preliminary tests in string_test.clj.

Use GLib for character classifications for now.

Documentation strings were taken from Clojure 1.5.0.
Add Pattern, re-pattern, re-seq, re-find, and re-matches.

Add initial support for regular expressions via PCRE.