package: update nan to 0.8.0, support node@0.11.9+

(Editor's note: but not node.js master, that will have to wait until
nan 0.9.0 is released.)

Fixes #88.
package: 2.0.7 -> 2.1.0

The minor bump is because of commit 053dfee ("lib: add convert/write/end
encoding argument") which is arguably an API change, albeit a minor one.
package: add tap dependency and `npm test` target

Follow the semi-standard practice of having a `npm test` target that
- surprise, surprise! - runs the test suite.  Should make the project
a little more approachable to outside contributors.