Remove Rails dependency

In the process, convert tests to use RSpec instead of Rails testing
framework
Add readline support for use with guard
Convert bing-ads-api_spec to rspec and clean it up
Fix campaign operations tests in campaign_management_spec
Fix ad group operation specs in campaign_management_spec
Add spec for getting multiple campaigns by account
Add spec for getting multiple ad groups by ids
Fix the remainder of campaign_management_spec
Add delete_campaigns to campaign_management
Automatically delete all campaigns after specs are run
Remove RDoc
Minor cleanup to campaign_management_spec
Clean up customer management spec
Clean up data_object_spec
Extract performance report request specs from report_request_spec
Extract other report request types from report_request_spec
Extract column helper spec
Extract filter helper specs
Fix reporting_spec
Extract creation helper functions in specs