Rename utility methods
Remove class_eval wrapper method
Cleanup fixtures
Refactor Twitter::API::Timelines#retweeted_by_user to use retweets_from_timeline
Add Twitter::API::Arguments class; remove extract_options! helper method
Ensure credentials set in Twitter.configure are of a valid type.

This commit adds basic credential validation after the credentials
are set using the Twitter.configure block. If the credentials are
not a string or symbol, a Twitter::Error::ConfigurationError is
raised with a helpful message.
Update ConfigurationError message to state that Symbol is allowed.
Delegate Tweet#profile_image_url and Tweet#profile_image_url_https to Tweet#user

Closes #340.
Only install pry-debugger on MRI
Fix timeout bug; lock faraday dependency to version < 0.10