Changed the way environment variable are parsed.

Previously environment variables where parsed using a prefix which was
generated from the utility name if not specified. I removed this
constraint and now flags are fetched by uppercasing the flag name and
replacing hyphens withunderscores, e.g. the flag "-rate-limit" will
translate to the environment variable "RATE_LIMIT".
Merge branch 'master' of https://github.com/namsral/flag

Conflicts:
	README.md