Adding in support to get attributes by attribute

Maybe I am doing it wrong, but there are a lot of cases that I want to get say a tracker or custom_field by name. Since the Redmine API does not support this it makes it difficult and you have to essentially fetch all records and then filter them down manually.

This should essentially do that process instead of having the api user write their own function

Does it make sense to support a non-lazy operation though?
Instead of using a list, would a generator make sense here?

e.g. `yield item`