Trim dependencies

Remove accidental use of gotest.tools instead of testify for asserts in the tests.

Improved use of testify/mock

Adds a new constructor so that testify/mock objects have the t object set on them, ensuring that if a server receives unexpected calls that it doesn't merely panic, but also fails the test.

Added Header Support

NewServer will now detect if the argument supports a HandleWithHeaders call, and calls that instead of Handle if it does.

Added go module support

v1.0.1

Added go.mod/go.sum

Initial release

v1.0.0

Update README.md