Add BasicAuth Realm support

[nshmura]

Hi,
Thank you for great library!

I want to split urls into two area. like this:

 http:// _ _ _ .com/admin/
 -> admin area

 http:// _ _ _ .com/super_user/
 -> super user's area

and each area has different id and password.

Most web browser manages id and password by domain and realm.
So if domain and realm are same, one pair of id / pw will be saved in browser, and another pair of id / pw will be forgotten.

I hope to BasicAuth middleware can change realm for this situation.
Would you please check this Pull Request?

[coveralls]

Coverage decreased (-0.1%) to 84.671% when pulling d28ea93 on nshmura:master into 70be2c2 on labstack:master.

[vishr]

@nshmura Thanks for your contribution 🎉

[nshmura]

@vishr Thanks! ;)