Support for single layer PSD's:

Before image.format('jpg', nil) on a single layer PSD yielded an
'unable to format' error. In the end it was only raised because the new
path wasn't guessed right.
Now it uses a regex and looks immediatly on the filesystem for the
correct file.
Add the RSpec binstub
Simplify selecting processors
Encapsulate Windows things in Utilities
Move require's into file that uses them
Fix case when processor is nil
Methods don't need "begin" blocks
Simplify MiniMagick::Image#read_character_data
Wrap comments to 80 columns
Fix imagemagick errors hanging instead of raised

Fixes #231
Bump to 3.8.1
Oops, we're using GitHub releases for changelog
Remove symbol access in info

To avoid any potential bugs when we saved a value as a string, but
request it later as a symbol -- it's all the same.
Remove an extra example of composition

Fixes #228
Merge pull request #243 from FlavourSys/master

Support for single layer PSD's
Make #format more robust and delete unused files

Fixes #236

Previously, when we would have an image in a Tempfile, and change the format,
the new file wouldn't be wrapped in a Tempfile anymore, which means it wouldn't
get deleted by the garbage collector. That means the size of the temp folder
would constantly increase, which is bad.

I decided to use `convert` instead of `mogrify` for conversion, because with
it I can control the output path. We generally use `mogrify` in order for changes
to happen in-place, changing formatting is the only place where `mogrify` acts
the same way as `convert` -- it makes a copy.
Merge pull request #248 from janko-m/more-robust-formatting

Make #format more robust and delete unused files
Remove MiniMagick::Image#destroy!

It only deleted the tempfile in case of an error, but the main beauty of
tempfiles is that they get deleted by themselves.

Another thing is that the name was misleading. `#destroy!` sounds like
it will delete the image in any case. But that's not the case; it won't
get deleted if `MiniMagick::Image` was initialized from a permanent file
(using `MiniMagick::Image.new`).
Tempfiles clean up themselves
Create a CHANGELOG to point to releases