parses the following types of byte ranges:
bytes=100-499
bytes=-300
bytes=100-
bytes=1-2,2-3,6-,-2
given an entity length, converts range to string
bytes 100-499/500
Based on RFC2616 3.12, 14.16, 14.35.1, 14.35.2
version: $version$ author: Helmut Hissen