Update README to reflect correct storage units.

Problem:

The prior example text in the README is incorrect:
* There are ~8 million bytes to the megabyte, not 8,192.
* The conversions library uses integer math; < 1 GB is reflected as '0'
  when the toGigabytes conversion is applied. The docs falsely imply
  that floating-point division is supported by the library.

Solution:
* Updated README.markdown to use examples that are not misleading, and
  that reflect the true output of scala interactive shell.