simplify logfmt

- logfmt is a constructor with the same properties as its prototype
- use lodash to cut down on `for` loops
- remove need to explicitly redefine functions on `namespace`
assert body parser stream when streams have ended
fix split module to not pipe empty string
Merge pull request #20 from jclem/style-refactor

This PR will clean up the style/syntax of node-logfmt and provide a number of bugfixes. The goal is a more idiomatic and readable style less prone to bugs.