Upgrade to Silvertip 0.4.1

Silvertip 0.4.1 contains significant changes compared to the previous
versions:

  - The 'Events#dispatch' method is removed. The control of the event
    loop is maintained in the application, which must call repeatedly
    'Events#process' or 'Events#processNow'.

  - The 'Callback#idle' method is removed. Keep-alives and such must
    be handled in the application between calls to 'Events#process' or
    'Events#processNow'.

  - The 'Events#stop' and 'Events#isStopped' methods are removed, and
    the 'Events#close' method is added.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #146 from jvirtanen/features/silvertip-0-4-1

Upgrade to Silvertip 0.4.1