Verify preconditions before revalidating

• Add a check which verifies if a precondition exists before revalidating
• Fixes issue #13
• Add regression tests
• Adjust one broken test

v0.4.1

• Update README.md

Simplify revalidation and ensure '*' validator works

• Modify 'revalidation' table so it doesn't check if the response is expired
• Ensure validator matching works with '*'
• Fix tests

v0.1.20

• Recently cached responses are cached correctly. Fixes issue #11

v0.1.18

• Ensure the 'Cache-Control: max-age' is validated
• Add regression tests
• Update documentation

v0.1.16

• Errors caused by empty HTTP headers. Fixes issue #8

No max-stale = allow anything

• No max-stale = allow anything. Fixes issue #7

v0.1.14

• Ensure Cache-Control is verified

v0.1.13

Don't cache the Host header

v0.1.12

• Allow newer rubies