Remove Rails 3.1 `read_inheritable_attribute` deprecation and add .rvmrc to gitignore.

Conflicts:

	lib/paperclip/storage/s3.rb
	test/storage_test.rb

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When an attachment has an uppercase extension and a default style, the to_s and url methods do not match when a size is not passed in. This contains the test and fix for this issue.

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I added lambda support to the size and content type validator, so now you can do something like:

<code>validates_attachment_size :avatar, :less_than => 2.megabytes, :message => lambda { I18n.t(invalid_image_size) }</code>

This is consistent with Rails validators, wich receive either a String or a Proc.

Regards.