Use secure URI for calling Skyscanner partner API

• Use https when making calls to Skyscanner API

Default error level changed to `GRACEFUL` and versioning chore

• Default error level is now graceful, it used to be strict.
• Version number should be set in one file only: skyscanner/__init__.py.

Support additional params for Flights Pricing Service

• As reported in #15
• Support for the following additional params during poll requests:

            'locationschema',
            'carrierschema',
            'sorttype',
            'sortorder',
            'originairports',
            'destinationairports',
            'stops',
            'outbounddeparttime',
            'outbounddepartstarttime',
            'outbounddepartendtime',
            'inbounddeparttime',
            'inbounddepartstarttime',
            'inbounddepartendtime',
            'duration',
            'includecarriers',
            'excludecarriers'

• Reference doc: http://business.skyscanner.net/portal/en-GB/Documentation/FlightsLivePricingList#pollsession

Fixes for version 2 and 3 Support

• Stable support for the other versions: 2.6, 3.3, and 3.4.
• Minor updates to unit tests.

First stable release

• Added support for XML responses. Return full response now + parsed content.
• Improved error handling and code documentation.
• Major code refactoring and improvement work.
• Updated existing tests to use current time and dates.

Thanks to Denis Dudnik (@ddudnik) for all these code improvements and stabilization.