Add ADD Register Encoding T2 support

This version of ADD allows access to the upper registers (R8 - R15).
This required implementation of registers past R12 which hadn't existed
in the simulator before.  I also added the ability for unit tests to
manipulate and verify these registers as well.
Remove instr field from PinkySimContext

I am not using this field so I removed it with this commit.
Document unimplemented features in README
Add an unpredictable instruction + fix addRegisterT2() decode.

The special data and branch exchange class of 16-bit instructions has
one encoding which is specified as being unpredictable.  Added tests to
make sure that pinkySim returns PINKYSIM_STEP_UNPREDICTABLE for this
encoding.

I also fixed a decoding bitmasking error in
specialDataAndBranchExchange() which sent the wrong bit patterns to be
processed as addRegisterT2().
Don't call pinkySimStep() from within CHECK_EQUAL

When the check fails, it ended up calling pinkySimStep() again to
display the result code in the failure log.