gen_client: running BOSH and new configuration options

As a result of efforts to make gen_client run over BOSH, there are now 2 new groups of options:

• connection - allows to specify type of connection and its parameters. Available types are tcp, ssl and bosh. 
• auth - allows to specify type of authentication and its parameters. There are currently 2 types reflecting exmpp authentication: basic (or legacy auth) and sasl. .

Note: this version of gen_client will not work with exmpp versions lower than 0.9.5.

Example: to login with my_acct@my_domain.com through BOSH served at http://my_server.com:5280/http-bind using SASL DIGEST-MD5:

gen_client:start("my_acct@my_domain.com", [{connection, {bosh, "http://my_server.com:5280/http-bind"}}, {auth, [{password, "my_pwd"}, {sasl, "DIGEST-MD5"}]}]. 

Yes, that's that simple. The default connection type is tcp, and default auth type is SASL PLAIN, so the code that uses previous versions of gen_client shouldn't break.

Note: as of now (v. 0.9.5) exmpp supports only SASL PLAIN, SASL ANONYMOUS and SASL DIGEST-MD5. Also, BOSH will only work with SASL, but not with basic authentication.