Ability to alias "pluck_attributes" column names

This new feature adds the ability to alias the keys (column names) of
the results returned from "pluck_attributes" (while still maintaining
the proper type-casts).

* Add tests for "pluck_attributes" to cover existing functionality
* Add tests for "pluck_attributes" to cover new alias[ing] functionality
* Minor clean-up/refactoring
* Update README.md
I prefer to use symbols for keys. By using a `HashWithIndifferentAccess` it still keeps the interface consistent for people already using a previous version of this gem and gives the flexibility to use symbols to address values (as well) w/ minimal overhead.

The more I think about it, since a user of the gem can `pluck` a value or attributes by a string or a symbol, I think it is perfectly reasonable to allow for the same behavior on the result.