NewWithDiskv function for creating a cache that uses an existing Diskv
Update README.md
Return errors from transport.RoundTrip
Factor out and export Date function (parses and returns the value of the Date header)
Merge pull request #11 from sourcegraph/NewWithDiskv

NewWithDiskv function for creating a cache that uses an existing Diskv
Merge pull request #12 from sourcegraph/handle-Roundtrip-errors

Return errors from transport.RoundTrip
Merge pull request #13 from sourcegraph/factor-out-date-func

Factor out and export Date function (parses and returns the value of the Date header)
change tests to use httptest.Server

instead of hardcoding port 9090, use the httptest package which will
automatically select an unused port.