Move specs to a rapgenius subfolder

`lib/rapgenius/foo.rb` should have a `spec/rapgenius/foo_spec.rb` file.
Add webmock and vcr as development dependencies
Add VCR configuration
Drop webmock requirement to 1.11.0

VCR spits out a warning about higher versions. It still seemed to work
but I hated seeing the message.
Update Song's spec

- Replaced `:fetch` stubbing with VCR cassettes
- Removed unneccessary testing of the `find` method.
- Broke some one-liner tests into `describe` blocks where there was a
  clear relation between multiple tests.
Refactor Scraper#url= a bit
Minor reorganization of Scraper; update Scraper specs

Introduces a 'Client' class that includes HTTParty and then sets some
sane defaults for all of the lib's requests. I kept the Scraper module
in tact and just updated it to use this new Client class so that
`include RapGenius::Scraper` in Song and Annotation still worked.

Where possible I kept everything the same for backwards-compatibility
until @timrogers wants to change things.

Also updated are the specs for the module. Removed some specs that were
basically testing low-level HTTParty stuff.
Update Annotation's spec
Merge pull request #6 from tsigo/vcr

Spec reorganization and updating