Added a linting task for gulp

I took the liberty and added a task `gulp lint` for the project.

To use the command: 

`npm install`
`gulp lint`

As a result, it'll give you a log like this:

```
[22:16:13] Using gulpfile ~/projects/javascript/mermaid/gulpfile.js
[22:16:13] Starting 'lint'...

/Users/jkim/projects/javascript/mermaid/src/graphDb.js
  line 58   col 18  'linktext' is already defined.
  line 131  col 33  eval can be harmful.
  line 144  col 51  eval can be harmful.

  ⚠  3 warnings


/Users/jkim/projects/javascript/mermaid/src/utils.js
  line 21  col 2  Missing semicolon.

  ⚠  1 warning

[22:16:13] Finished 'lint' after 276 ms
```