Remove .c files

Cython manual recommends adding .c files so anyone can build Cython
extensions but you can also really install Cython and do that. I don't
see the point.
Rename host2str.pxd to conversion.pxd and add funcs
Update gitignore
Add gdb* scripts for debugging

These might just not be useful for you at the moment. Basically you
need:

* Python build with --with-pydebug --without-pymalloc --with-valgrind
  etc. Also set CFLAGS to contain "-g -O0"
* Then use gdb and the gdb python extensions
* You may also have to rebuild Cython and this library
Bind LDNS constants to module root
Rename some errors and use custom message
Make setup.py more general
RR: Implement more features from rr.h
Packet: Add clone, fix segfault, more methods
Rdata: Add some casting features