Make it easy to define default or global settings (such as user credentials)

[wahuneke]

Make it easy to define default or global settings (such as user credentials) by attaching default values to the class instead of having to define the same values over and over in each test URL definition

Update documentation to explain the new 'defaults' feature

[coveralls]

Coverage remained the same at 100.0% when pulling 44538c8f317143cd08358d33e8cffdc5638544e7 on wahuneke:master into 263a6d09fe6088cd214111cd814e56b16b782853 on steelkiwi:master.

[bellum]

@wahuneke Hi Bill. Thank you for your PR. Could you also provide new tests for defaults feature?

[wahuneke]

Yes, I will work on updating the tests