StorageHTTP class and tests.

Closes #197.
Creates the StorageHTTP class and all the necessary methods:
* Create new Qabel Storage Volumes (QSV)
* Probe QSV
* Upload blobs to QSV
* Retrieve blobs from QSV
* Delete blobs from QSV or the whole QSV

Updates also the travis file for testing.
Some tests are ignored, because the Qabel Storage is not up to date with the documentation. I will open an issue in the storage repository. I tested it with the branch https://github.com/Qabel/qabel-storage/tree/protocol_update and some edits, so the tests will not work for the moment.

Also: Later the storage volume will be added directly to the settings, if desired.
Why adding the trailing slash? IMO the slash is not part of the public identifier.