Document how to cleanup after test runs

Adding an example for using Capybara/FactoryGirl to test attachments. I
ran into an issue on my project where the files I was uploading in my
test were getting saved multiple times, leading to hundreds of extra
files before I caught it. I did a bunch of research and found this to be
the most elegant solution (saving files in the testing environment to a
custom location and then deleting that directory after the suite runs).
I thought some clarification in the README would help people deal with
the more easily in the future.
Support per style options for s3 storage class
Use #to_s on the geometry option in the Thumbnail processor.
This should make it possible to avoid passing a :geometry in the style
definition (when passing it as a hash), thus making Paperclip avoid resizing the
image. This is helpful when you want to perform your own custom
resizing/cropping using :convert_options rather than the default options offered
by Paperclip itself.
Test allowing nil geometry
Merge branch 'master' of github.com:thoughtbot/paperclip into duplicate_error_messages