Moved unit test from mocha to lab

package.json
* changed devDependencies from mocha to lab

lib/sax.js
* commented out the console.log statements, as we don't want these to show up in the test output

test/document.js
* ported all tests to use the Lab syntax

test/test.xml
* added a (valid) entity block, which we should be implementing at a later stage

update Makefile to use lab, added 'test-coverage' and 'test-lint' instructions