get it compiling on ruby 2.0

Still not working due to `rb_vm_make_env_object symbol` lookup error
cf:
```
john@john-VirtualBox ~/ruby/binding_of_caller $ pry
[1] pry(main)> require './lib/binding_of_caller.so'
=> true
[2] pry(main)> binding.of_caller(1)
pry: symbol lookup error: /home/john/ruby/binding_of_caller/lib/binding_of_caller.so: undefined symbol: rb_vm_make_env_object
john@john-VirtualBox ~/ruby/binding_of_caller $
```
first attempt at debugger_api port
Tests mostly passing on ruby 2.0

Now using debug_inspector gem.
Work left to do:

* Binding#frame_type
* Binding#frame_description
* Convert C extension to use ArgumentError rather than RaiseError when out of band
frame number is requested

We can implement these using frame_iseq()
All tests passing on MRI 2.0

Added support for Binding#frame_type, via frame_iseq in RubyVM::DebugInspector.
Note it just appeared like the 10th element in iseq.to_a returns the frame type
but this could potentially be unreliable, though it seems to work fine. :D
ArgumentError -> RuntimeError
version 0.6.9pre1 release

* added support for Binding#frame_type, improved binding collection.
* pry-stack_explorer now passes its test suite using this gem
remove ruby 2.0 headers

no longer need these as we're using debug_inspector gem instead.