gdb: support qAttached command from debugger

When using avr-gdb on Ubuntu 12.04, this command is always sent
and we responded as if it was unsupported. Unfortunately avr-gdb on
Ubuntu 12.04 crashes when detaching from the process because it makes a
bad assumption somewhere, this will prevent the crash.
gdb: support qOffsets command

GDB will ask what offsets should be applied to the the text and data
sections. Since AVR is Harvard architecture so we need to apply an
offset for one of the regions universally, Atmel documents and uses
0x800000 for RAM so this makes that happen.
gdb: support reporting memory map locations

GDB wants to have knowledge of the RAM and flash regions of memory on
the chips. Since AVR is Harvard architecture, the RAM must begin at a
fake offset and per Atmel's docs that is 0x800000.
Merge pull request #65 from cardoe/gdb-stub-update

Add some commands to the GDB stub