Original documentation was largely an exact copy of cortex-m-semihosting
at the fork. This commit fixes that, removing unnecessary references to
cortex-m-semihosting and clearing out the changelog so that it's
relevant to this repository instead.

Merge upstream cortex-m-semihosting

Without this the correct sequence is not triggered in OpenOCD and is
treated as a regular debugger break.

While here update to use asm!() syntax.

refs: http://www.keil.com/support/man/docs/armcc/armcc_pge1358787051159.htm

Stdio fixes after review

See #11.

drop write macros in favor of IO objects + fmt::Write

closes #4
closes #5
closes #6
closes #7
closes #11

watch tail -> tail -f

note that it's possible to skip the separate log file.

closes #9

Add `exit` and `report_exception` syscalls.

add a macro to write to the host stderr

also fix `write_all` logic when the a single write doesn't write all the bytes

fix typo in hprintln!

needed to cargo publish