check keystore size in MobileidSign.java and soap message creation and per call key-/trust-stores

[ralfhauser]

https://github.com/phaupt/mobileid-java/blob/master/client-saaj/src/com/swisscom/mid/client/MobileidSign.java should read the keyStore-File and check whether it's size is really 1 (do to the not that usual implementation on their server side).

Also, you might want to mention in createSOAPRequest() that this doesn't work when you have other libraries such as AXIS-2/axiom on your classpath
(maybe mention https://issues.apache.org/jira/browse/AXIS2-5942 that solved it for me)

Thirdly, please mention that keyStores/trustStored per soap-call (instead of globally for the entire JVM) can be implemented with the hints of the same AXIS RFE.