Moved contrast.Adjust to Linear, added new Adjust func.

The new function uses the formula in ImageMagick, but allows you to
set the value. This is the new default.

Moved the old linear function version to contrast.Linear as it made
the most sense.
Update for new definitions
Simplified README. Removed lots of contents.

Reason: I think it was trying to do too much. You can easily find
out about the tools by running help, which is going to be the most
up to date.

The README now just gives a broad view of how to use img on the
command line and as a library. The section on External Scripts
may be changed further, in the future.