Mark EnginioClient properties as final.
Remove EnginioClient constructor that creates ugly code.
Remove initialized property from EnginioClient
Mark EnginioIdentity as internal

The class has no value for end-users
Change type of backendId and backendSecret properties to QByteArray

Internally we need QByteArray so the change reduces amount of
QString -> QByteArray conversions.
Merge pull request #87 from nierob/changesAfterFirstReview

Changes after first review