Added ability to get photos based on userName

[oisinwood]

• Example Usage
  • $(".instagram").instagram({
  • userName: 'oisinwood',
  • accessToken: '8405067.03a31f3.a178667f364d4243a3523803a35a55af',
  • show: 6
  • });

[potomak]

I like this feature but I think it's a bit confusing to get userId by userName and consequently photos of that user by userId.

If you want me to merge the pull request please just expose user search feature.

Thanks.