Update README

Minor corrections and updates to better align with recent changes to
make MRI a submodule and to use non-recursive makefiles.
Add more packet tests related to checksum digits

Make sure that the packet handling code can handle invalid and
uppercase hex digits.  This gets the code up to 100% code coverage.

I also updated the extractExpectedChecksum() function to catch invalid
hex digit exceptions and handle them as invalid checksum errors.
Initial commit for MemorySim module

This module provides an IMemory implementation which can be used to
simulate READWRITE and READONLY memory connected to pinkySim.  I will
add more functionality to it soon.

This commit also contains makefile cleanup:
* Delete .gcda files when rebuilding .o object files so that stale
  coverage data isn't left kicking around.
* Add libmocks library which I used for introducing out of memory
  faults into my MemorySim unit tests.
* Automatically add pre-reqs to make targets like 'all', 'test', 'gcov'
  etc. to reduce the number of places the makefile needs to be touched
  in the future when adding new modules.