Various fixes

[eltos]

Replaced byte string concatenation with bytearray extend functions.
This is ~50 times faster!
(See also #8)

[eltos]

The additional commits include fixes for:

• Exception when trying to close a lost connection, resulting in an unability to reconnect
• Timeout parameter not used for creating the core client when opening a connection, causing the port mapper routine deep in the subsequent calls to block infinite under cerain circumstances