feat(log): add logging

Importing the log package from github.com/kelseyhightower/confd/log
allows us to have control over the log output of our own logs.
It also has a nicer interface to work with than the default log
package in go.
fix(tests): test the doc package

Right now, no other go packages exist in the project, so the testing
framework will continue to cry on TravisCI without this fix. This
explicitly tells make test to run the tests under the doc package.