Fixed parsing WindowsPhone message statuses.

Added "NotAvailable" enum fields to better match documentation, enum
parsing changed to case insensitive (so it now correctly parses value
"Inactive" to "InActive" field).
Changed Encoding from Default to UTF8

Windows and WindowsPhone notification payload was encoded incorrectly.
For notifications with international (polish in my case) characters
Notification Service returned OK status, but notification didn't appear
on the device.
After changing to UTF8 it works as expected.
Merge pull request #102 from billennium/master

WindowsPhoneMessageStatus parsing and payload encoding fixes

Thanks for contributing :)