v0.2 - Fix problem when running on battery power

The primary reason for a v0.2 is that this solves #12, a hang on battery power. There are also major bug fixes on integer arithmetic, and a new formatting of numbers.

Full Changelog: v0.1...v0.2

v0.1

First preview of the firmware with pre-built binaries

To install:

1. Put your DM42 in USB disk mode
2. Expand the contents of the release at the base of the disk. You should have four files, DB48X.pgm, DB48X_qspi.bin, help/DB48X.md and STATE/test.48S
3. Using the calculator's setup menu, flash QSPI image from USB disk. This will load DB48X_qspi.bin and erase it from the disk.
4. Using the calculator's setup menu, load the DB48X.pgm.

Note: Normally, the QSPI binary is "compatible" with the original DM42 program, so that you can switch back and forth. There may be limitations if the symbols in that file are not at the right address. If your DM42 program misbehaves after installing this QSPI, restore the original QSPI image from SwissMicros.