Added merging of dynamic fields on ticket merge, thanks to Moritz Lenz.
Squashed commit of the following:

commit 62d77332c8108ccf28901b61611e81026de7cbf8
Author: Moritz Lenz <mlenz@noris.net>
Date:   Fri Mar 7 16:11:36 2014 +0100

    Implement merging of dynamic fields on ticket merge

    For a list of configured dynamic fields, in case of a ticket merge,
    the value from the merge ticket is copied into the main ticket if
    the main ticket has no value set.

commit 93fb652084274bc1385cde11464e6186b75027c7
Author: Moritz Lenz <mlenz@noris.net>
Date:   Sun Mar 9 08:37:15 2014 +0100

    Fixup config