Integrated the latest version of bouncycastle.

* BC 1.48 added a check to throw an exception if a non-base64 character is
encountered. An OTR message consists of ?OTR:AbcDefFe. (note the terminating
point). Otr4j doesn't strip this point before passing the content to the
base64 decoder - buuum. This is why .decode(content.substring(0, content.length() - 1).getBytes())) is needed.
Changed some method signatures in OtrEngineHost. Added new method in Session which allows users to check whether SMP negotiation is in progress