Fix for thread-safety issue

It seems SSLSocketFactory (which is used by the BaseTransferAPIClient) is not thread-safe, which can cause problems when sending requests at the same time (code to show the problem is in src/main/java/org/globusonline/transfer/ExampleParallel.java).

In order to get around the issue I added an option to enable multi-thread support for the class. Didn't want to make it default because it introduces a  performance hit for single-threaded usage.

Also added constructors for JSONTransferAPIClient & BCTransferAPIClient that enables direct use of Key-/Trustmanagers instead of proxy file(s).