Make SharedAccessSignatureTokenProvider capable of handing updated creds

(New issue based on point 4 in issue #55.)

Consider making SharedAccessSignatureTokenProvider allow updating the credentials. Something similar to this pseudo-code:
``` java
public class SharedAccessSignatureTokenProvider extends TokenProvider {

...

public void set_sharedAccessSignature(String value) { }
public void set_keyName(String value) { }
public void set_sharedAccessKey(String value) { }
```