Add docs badge to README

There's nothing they can be spoofing, and there's little danger of a
webserver accidentally sending a dangerous Content-Type with a file that
has no extension. If the browser chooses to interpret it in a dangerous
manner, no server-side library on earth can change that.

This isn't specifically related to RSpec, but is built off the branch
and is part of an effort to get the CI build green.

First off, sqlite3 needs to be exactly v1.3.8.
Need to make sure both racc and rubysl are in the Gemfiles.
RBX's File.join doesn't like nil at all.
RBX also doesn't like to remove a constant that doesn't exist yet.

There were a bunch of free-floating methods in spec_helper. They were
pulled over from test/helper, but they shouldn't be there. Move them
into appropriate modules in spec/support. This commit also cleans up the
"should_accept/reject_dummy_class methods in favor of using proper
matchers directly. This commit should probably not get squashed or git
will likely forget that those matcher specs are the same file because of
how much changed.

This is a big one.

This change converts all of the existing tests from test/unit to RSpec.
It's been a long time coming, and was specifically prompted by the
inability of minitest to work with Appraisal across the various Rails
versions. Instead of ironing out that headache, converting to RSpec was
preferable (after initial probing).

Many assertions are still assertions and are housed in the Assertions
module, but they should be converted as they get touched. Similarly,
since we're touching all the tests/specs, we convert all the hashes from
1.8 syntax to 1.9 syntax.

Typo in readme

Fix typo "defauly" -> "default" in NEWS

The `file` command on OS X takes the --mime-type option, which returns
only the type and not the optional encoding. This is nice, but it's not
portable. Use --mime and strip the encoding off if it's there.

And don't forget this time.

Validators love instances. Seriously, though, this was causing a problem
when the validator was being called and the private "send" didn't have
an instance to work on.

Update README to v4.1 for installation

This will make Travis not waiting for all builds to finish before notify
us the build result.

http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

Update link to paperclip-meta repository

Ownership of the paperclip-meta repository was transferred from @y8 to @teeparham.

Gemfile.lock.