Added new Voice API call methods

Added Application API methods

Added check_signature method for checking callback signatures

Deprecated old Verify API methods

Added get_sms_pricing method

Added get_voice_pricing method

Added get_event_alert_numbers method to get opt-in/opt-out numbers

Added resubscribe_event_alert_number method to opt-in a number

Added more clearly named methods for Verify API

Added app_name and app_version options

Added topup method

Added update_settings method

Added api_host attribute

Added ClientError and ServerError classes

Moved repository to https://github.com/Nexmo/nexmo-python

Added get_basic_number_insight method for Number Insight Basic API

Added get_number_insight method for Number Insight Standard API

Added User-Agent header to requests