Fix warnings of rack session cookie.

Add a secret in tests so we get no deprecation warning.
Remove unnecessary tests.

I was being over zealous with testing previously.
Fix for 1.9, 2.0, etc.

Drop PBKDF2, use OpenSSL built in PBKDF2 checks instead.

In addition, use an eql_time_cmp check which protects against timing
attacks.

More info here:

    http://www.ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/PKCS5.html
Rename time compare function, add docs.

Add a link to the original web URL specifying the usefulness
of eql_time_cmp.
Remove outdated examples.

TODO: Add a shield-sample-app.
Reorganize tests.

We're going retro here and doing the _test.rb convention.
Allow customization of the hash function.

Just an attempt at cross-platform-ness.
Make PBKDF2 work.
Merge branch 'vendor-pbkdf2' into refreshment

Conflicts:
	lib/shield.rb
Works with PBKDF2 gem again, with just a minor patch.

Latest resolution: release a fork of PBKDF2 that has the fixes
and also has cleaner code.
Rename test to tests for `make test` to work.
Use armor for PBKDF2.
Use Armor.compare for constant time string comparison.
Merge branch 'refreshment'

Conflicts:
	lib/shield.rb
Use Armor.hex for generate_salt.