added de-/serialization of multi sub-keys of QblPrimaryKeyPair
added de-/serialization of multi sub-keys of QblPrimaryPublicKey
added multi sub-key support to class Contact:
- new getter for retrieving list of sub-keys
- marked old getter @Deprecated
- changed name of sub-key setters from setX to addX
changed modifiers of generateEncKeyPair() and generateSignKeyPair() to public
changed contactTest() for testing multi sub-key serialization
changed syncedSettingsTest() for testing multi sub-key serialization
Update travis.yml for storage testing
Adding StorageHTTP and StorageHTTPTest classes
Add new method to create new Qabel Storage Volumes

Add also a HTTPResult class, which contains the response code and a boolean,
if the request was successful.
Add a method, which tests the new function.
Add the probe method and the corresponding tests
Add upload method for blobs to the QSV
Add method to delete blobs or whole storage volumes
Change to the updated qabel storage branch
Rename file to blob
Tidy up after every test.
Removed generic parameter from DropSerializer
Remove slash from public identifier in StorageVolume.

Adding the slash in every StorageHTTP method
Change data variable to generic
Removed redundant generic parameters from test DropMessageGsonTest methods
Introduced as()-method in ModelObject to syntactically work-around casts e.g. when using DropMessage.data objects