Fixes and improvements

	⁃	Separated the library (functions) from the testing, so functions can
be tested one at a time, and so a dev can include Oauth.php in a
project without having to remove test lines at the end. Simple testing
page added which includes the library.
	⁃	Allow author to specify their own app name instead of "jonhurlock
Twitter Application-only OAuth App v.1". Because it is the author's
app, they are using jonhurlock's library in their app.
	⁃	Retrieve http code AFTER you make the curl request
	⁃	For consistency and readability, under "get_bearer_token()",
"$bearer_token" and "$base64_encoded_bearer_token" variable name
changed to match other functions, and because they are not bearer
tokens.
	⁃	Content-length header is unnecessary, it is added automatically
	⁃	adding '.""' after a variable in $headers array is unnecessary
	⁃	remove "$header" variable before curl option declaration, unnecessary
	⁃	"invalidate_bearer_token()" and "search_for_a_term()" return a php
associative array instead of raw http data.