Needs some refactoring and some optimization, but fixes several bugs.

Now that I'm using the step strategy, I put in an optimization where if
stepping into another function, simply find the return address, put a
breakpoint there, and then continue.

I decided to vendor all debug/dwarf and debug/elf files so that the
project can be go get-table. All changes that I am waiting to land in Go
1.4 are now captured in /vendor/debug/*.