0.9.0: Merge pull request #10 from mdb/support-ca

• addresses issue #3 to support the ability to configure a ca
• addresses issue #8 to improve check behavior
• addresses issue #7 to improve documentation
• README.md clarifies that only host and key are required source fields
• introduces various code improvements to ensure errors are better handled
• introduces some helpful metadata on various actions
• introduces additional tests that further exercise concourse-consul-kv-resource functionality