added `keep_empties` option to `split`
ported pub/sub unit test to new interface
changed displayed unit test names: {test__=>test_}
changed LGPL version to 2.1
replaced license text with LGPL 2.1
added const to local_actor::joined_groups
removed handle_sync_failure from public API
let stacked actors execute `become`
use namespace std in hello world example
Merge branch 'topic/opencl' into unstable
fixed requirement in response_handle
beautified OpenCl examples
changed test for opencl actor

Test spawn_cl with and without map functions.
spawn_cl can now handle opencl::program

The kernel source can now be passed to spawn_cl in form
of a cppa::opencl::program. A test for this was added to
test_opencl.
maintenance and documentation

{apply_args => call}.hpp, added new function: `call_mv` (calls a function
and moves all arguments), and documented spawn_cl function family
print errors and warning if compiled w/o logging
fixed logging (replaced wrong log macro calls)
become no longer execs self->exec_behavior_stack

the automatic invocation of self->exec_behavior_stack unfortunately
breaks `sync_send(...).then(...).continue_with(...)`
used limited vector for OpenCL dimensions

OpenCL interface now uses vectors with a maximum size of 3, because
the dimensions settings of OpenCL assume a one, two, or three dimensional space
maintenance

removed unnecessary moves and enforced coding conventions