doc: typo in doc/KERNEL.md
core: check for GPU_* environment variables unconditionally.
doc: add releases, issues and irc channel to README.
ui: remove mention of cgminer in "Settings" menu.
kernel: search in `pwd`/kernel/ for kernels.

Closes https://github.com/veox/sgminer/issues/49.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	sgminer.c