take advantage of ::None for identifying unreachable code
Merge branch 'master' of github.com:JuliaLang/julia
rename backtrace() to catch_backtrace(), since it gives the backtrace
of the error you caught in the current catch block

make backtrace() give a trace from where it is called

expose the function that maps code addresses to source location info

closes #1952
Merge branch 'master' of github.com:JuliaLang/julia