Added node-syntaxhighlighter to dependencies.

Adding node-syntaxhighlighter as it's required in [lib/server.js](https://github.com/qjcg/imdone/blob/master/lib/server.js#L11-L12).

When node-syntaxhighlighter is not installed, the user gets this on invocation :

```sh
$ imdone -o

module.js:340
    throw err;
          ^
Error: Cannot find module 'node-syntaxhighlighter'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/usr/lib/node_modules/imdone/lib/server.js:11:15)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
```