Added missing gulp-task p-lib:dev:reload to gulpfile

Thank you for this great generator that I'm sure will help me get of to a flying start with polymer.

I've run into a small problem. When running `gulp serve` and changing files in the `.bower-components` dirs the gulp sequence stops with the error:

```
Task 'p-lib:dev:reload' was not defined in your gulpfile but you tried to run it.
```

It seems that this task is missing from the gulpfile. This pull-request should fix this issue.