Return 'user.User' when listing all of the users of a company

[tanasegabriel]

Fix for #180
We're incorrectly returning users belonging to companies as company.Company objects and this can cause issues if trying to update the user right away, unless an extra call to list the user is used - this PR addresses this.

Before:

>>> for user in intercom.companies.users(company_id):
...   user.custom_attributes["newCDA"] = "yup"
...   intercom.users.save(user)
...
<intercom.company.Company object at 0x7f109be4d710>

>>> for user in intercom.companies.users(company_id):
...   print(user.__class__)
...
<class 'intercom.company.Company'>

After:

>>> for user in intercom.companies.users(company_id):
...   user.custom_attributes["newCDA"] = "yup"
...   intercom.users.save(user)
...
<intercom.user.User object at 0x110ba8898>

>>> for user in intercom.companies.users(company_id):
...   print(user.__class__)
...
<class 'intercom.user.User'>