Juju provider for CloudSigma - client (for review)

This pull request is part of juju provider for CloudSigma contaning the following files:
- provider/cloudsigma/client.go
- provider/cloudsigma/client_test.go

See the incremental diff here - https://github.com/Altoros/juju-cloudsigma/pull/1

Provider for CloudSigma (https://www.cloudsigma.com) uses CloudSigma API v2.0 (https://cloudsigma-docs.readthedocs.org/en/2.10/).

Minimal client for communicating with the CloudSigma Web API from Go program was implemented as separate library https://www.github.com/Altoros/gosigma. This library is published under AGPLv3 license.

Actually, waiting was required here, because at the end of StartInstance method we move our local temporary storage to bootstrap machine, that's why this machine should be running, after StartInstance method was called. This behavior was refactored in  cloudsigma_provider_simplestreams branch. Waiting was removed and code, that moves storage was added to finalizer return value of Bootstrap method.