Defers web requests until they're actually necessary to fetch information

Adjusts tests to reflect changes to how web requests are made

Removes "defer_prefetch_init" arg from YouTube object initialization

Converted most attributes into caching properties to facilitate request deferring.

Added some documentation to uniqueify to explain its purpose.

Prefetching added to conftest to improve test speed

Added some setters for YouTube properties.

Some cosmetic README changes