Add Makefile and 'release' task.

This PR adds a `Makefile`, which includes a `release` task to do the following things:

- make a new git tag with the current version specified in `package.json`
- push the new tag to GitHub
- checkout the `osx-node-pre-gyp` branch, merge master into it, and push to GitHub (to tell Travis to make a new binary)
- publish latest master to NPM

Example invocation:

    $ make release
    Tagging release 1.3.1
    Pushing tags to GitHub
    Switching to osx-node-pre-gyp branch
    Merging master into osx-node-pre-gyp
    Pushing osx-node-pre-gyp
    Switching to master branch
    Publishing to NPM

This is 3rd PR referenced in [this comment](https://github.com/voodootikigod/node-serialport/issues/290#issuecomment-39625224).