Add `assign` to prospect accounts

'assign' is actually not included in the documentation, but I found that it works for not only Prospects but Prospect Accounts as well.

- Added spec for 'assign'
- Added code for 'assign' in ProspectAccounts object.

Additionally:
- Modified meta-programming approach for #read and #update so that the code is more self-documenting and IDEs include the methods in auto-complete.
- Modified the object instantiation on line 5 to include parent modules too for clarity.