Allowing import without setting environment variable for ID

[wxcutch]

In response to issue #112

Anything outside of the class keyword in python code gets called statically upon import of that script. DomainApi(get_client) was being called when importing domain.py and, by extension, fastfuels_sdk. (This occurred in multiple instances, domain.py is just an example.) There's never an instance where statically calling the API is necessary, so I put the api calls in constructors for all the classes and called for the API within the one static function, that way the api's are only fetched when necessary.

[amarcozzi]

Sorry for the delay on this @wxcutch. I did something similar to this on PR #139 which should fix both #112 and #98. I'm closing this PR.