Allow to specify Akamai API host.

Added an argument to PurgeRequest constructor, which allows to
override the Akamai API host that will be used for requests.
Make it possible to control SSL cert verification.

Added an argument to PurgeRequest constructor, which allows to
configure how SSL certificate verification is performed when doing
HTTP requests.
Merge pull request #7 from elmirjagudin/api-host

Make it possible to specify API host per PurgeRequest