Fix issue #8.

In the Python version of this code, there's a wrap_ord function that converts
characters into numbers. In lib/rchardet/jpcntx.rb there's no mention of this
function and we're trying to simply take numbers away from chars, which does
not work. Fixed this so that we're subtracting from the character's ordinal
number.
Merge pull request #9 from invisiblehand/master

Fix issue #8.