Adding support for using delegated GSSCredential for Kerberos authentication

Internally htmlclient relies on GSS API which uses JAAS configuration
specified by user to get the GSSCredential. This patch will allow a user
to avoid the config file and directly passing a delegated or normal
GSSCredential.

Snippet to set GSSCredential for SPNEGO-KERBEROS Authentication :
//gssCredential is the GSSCredential Object
KerberosCredentials kerebrosCredential = new
KerberosCredentials(gssCredential);

CredentialsProvider credsProvider = new BasicCredentialsProvider();
credsProvider.setCredentials(new AuthScope(null, -1, null),
kerebrosCredential);

Registry<AuthSchemeProvider> authSchemeRegistry = RegistryBuilder
.<AuthSchemeProvider> create().register(AuthSchemes.SPNEGO,
new SPNegoSchemeFactory()).build();

Use this authSchemeRegistry for HttpClient.