Add engines requirement for npm >= 1.2.10

As suggested by @sindresorhus. This should at least give a hint to the users if they install the generator in an outdated environment. The warning they see looks like this:

![screenshot from 2013-06-25 11 12 59](https://f.cloud.github.com/assets/9906/701512/7f9820ac-dd77-11e2-8608-488da71b034c.png)

Related: #94