Added correct converstions for PB and EB.

 * We were converting Exabytes to bytes as Petabytes.
 * Updated tests to cover ever unit.
 * Added note that we're going by the old, inaccurate definitions of
   Kilobytes, Megabytes, etc, in that we treat them as powers of 2.
Merge pull request #343 from zacharyalexstern/fix_to_bytes

Added correct converstions for PB and EB.