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/*.

* Fixes incorrect loop `next`ing behaviour
* Includes fix for determining return address

Improvements:
* `next`ing through a loop works correctly (when not already within a loop)
* `next`ing out of a function works correctly

Needs work:
* `next`ing in a loop can be improved when starting within a loop

This current implementation does not cover the following:

* Setting correct breakpoint when exiting loop
* Setting correct breakpoint when returning from function
    * All facilities are available for this, it just is not taken into
      account in the current `next` implementation.