Make all exception classes inherit from std::exception

This refactors out a very common pattern of formatting "%s: %s" with
e.function.c_str() and strerror( e.the_errno ) into just the what()
method of NetworkException. It's also a prerequisite for making cleaner
public API for any exceptions we throw, and allows us to more easily
get exceptions passed back to us to handle.

src/examples/decrypt.cc

@@ -62,7 +62,7 @@ int main( int argc, char *argv[] )
 	     (long)message.nonce.val() );
     cout << message.text;
   } catch ( const CryptoException &e ) {
-    cerr << e.text << endl;
+    cerr << e.what() << endl;
     exit( 1 );
   }