Introduce faked network replies

API depends on JSON which can be missused in many ways, sometimes
the library knows, without connecting to a backend, that a request
is wrong. In such cases it is nice to get an EnginioReply instance
that can be queried instead of a null value.
Add missing EnginioReply::isError function
Fix qml identity test.
Use faked reply for invalid search request
Move identity tests to a separate file
Imporve EnginioFakeReply.

EnginioFakeReply name seems to be better then
EnginioFakedReply.

EnginioFakeReply::bytesAvailable function is not needed
as the reply is not sequential

Fix potential problem of reading not through readAll
function
Merge pull request #105 from nierob/fakedReply

Introduce faked network replies and small fixes