Return the default path when the attachment has no remote file

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Using the ActiveRecord validations is smart in order to lessen load on
the project to develop our own validators, but it's problematic in that
calling `valid?` on the record fires off all the other validations --
including those on fields which may not be set yet because of
mass-assignment. This commit will "pre-validate" the attachment's fields
so that it doesn't process an invalid attachment, but it does so by
running its validations manually on assignment.

Refactor Paperclip::Attachment to eliminate code duplication between #extra_options_for and #extra_source_file_options_for

This commit makes this method move by default instead of copying, which
should make it perform significantly better with large files. It falls
back to the old semantics if the mv fails.

Fixes a typo in the README.md

Thanks to ericsullivan for pointing this out.

This propagates attachment size and content_type validations to the main
attachment attribute, to make them behave the same as presence
validations. This allows paperclip to play more nicely with simple_form
and other form abstraction gems.

This commit includes a behaviorial change. If you emit all errors on a
model instance you will see some failed validations twice: once for the
base attachment attribute and again for the failed presence,
content_type or size attachment attributes.

Merge branch 'rake-task-exception-handling' of git://github.com/holetse/paperclip into holetse-rake-task-exception-handling

There was a spec that was failing because we removed .attachment_definitions
from models, I expected this to fix it, but apparently it hasn't.

https://github.com/thoughtbot/paperclip/issues/760

This allows bundler to complete gem dependency resolution during
development. This is only relevant for paperclip developers. Please see:
https://github.com/thoughtbot/paperclip/issues/1256 and thanks to
@nkondratyev