parallelized read image and PSL conversion using 3 processes by default.
added shebang. removed specific matplotlib backend.
removed paralellization, because numexpr gave an even better speedup.

If numexpr is not installed, the code will fall back to the single
threaded (slow) numpy version only.
removed some warnings, minor improvements.

removed 'RuntimeWarning: Invalid value encountered in less'
added -l and -s option.
added colorbar and --log
Merge branch 'dev'

Conflicts:
	ipread.py