Add HTTP properties to generic error

Allows generic HTTP error to accept more detailed information about the response from the server.
Throw more detailed generic HTTP error

Take advantage of new response code and body properties to allow better logging.
Preserve lack of line endings.
Add Mailgun->verifyWebhookSignature
Merge branch 'master' of https://github.com/nathanntg/mailgun-php into nathanntg-master

Conflicts:
	src/Mailgun/Connection/Exceptions/GenericHTTPError.php
Merge branch 'nathanntg-master'
Merge branch 'verifyWebhookSignature' of https://github.com/gmacon/mailgun-php into gmacon-verifyWebhookSignature

Conflicts:
	src/Mailgun/Mailgun.php
Merge branch 'gmacon-verifyWebhookSignature'
Merge branch 'master' of https://github.com/mailgun/mailgun-php into travis/merging