Add catch, throw

attempt to write cps transformations as described here

• src/anglican/emit.clj: wraps query body inside a try

• src/anglican/state.clj: add field ::catch-stack to state and corresponding methods to modify

• src/anglican/trap.clj: cps transformations for catch, throw ; dispatch methods

• test/anglican/trap_test.clj: very simple unit tests for cps transformations of catch, throw

• test/anglican/catch_throw_test.clj: contains tests for catch-throw functionality