Add rake to the test group

Without this `bundle exec rake` errors out
Add rank for empty hands. Fixes #1

Hands with no cards in them now return a rank of "Empty Hand".
Mention ruby 2.0 support in README
immaterial variable name change
Simplify Card class to fix hellacious bug

The logic is some of the Card constructors, notibly build_from_value and
build_from_face_suit_values was overly complicated and under tested. The
result was a bug where the Card#value did not match the corresponding
suit and face value.

This commit changes the calculate of value (simplifies it a lot). The
value should be internal to the Card class but this is a breaking change
if anyone was depending on it to be stable.
Validate library against literally a million poker hands.

Test script is run at will because it takes a long time and requires
downloading 23MB file.