Bump version
Fix segfault on Color.{red,green,blue}
Add Color.alpha property
Make Image.transparentize() to work

Also: add a missing test for transparentize(), and change a way to
test Image.watermark() from comparing two signatures to comparing
affected pixels.
Change way to test watermark

It now tests only whether it is affected, not particular resulting colors.
It's more portable way working on more various environments.
Remove trailing whitespace
Fix bug make_blob() didn't work after convert()

Fixed a bug that format converting using Image.format property
or Image.convert() method doesn't correctly work to save blob.
Release 0.2.3
Merge branch '0.2-maintenance'