Allow scoped actors to use `planned_exit_reason`

This patch allows scoped actors to define a desired exit reason using
`planned_exit_reason` to cover use cases as mentioned in #131.
Switch to Boost Software License 1.0
Change version to 0.9.3
Remove LGPL reference from README
Remove `self` from docu for class-based actors
Fix error when building with logging enabled
Remove `exit_reason::unallowed_function_call`

The exit reason `unallowed_function_call` is a remains from older versions of
libcppa and is no longer used.
Re-implement unit test for local groups
Adjust warnings, fix #132

This patch downgrades all warnings that could trigger in valid use cases (as
shown in #132) to infos.
Downgrade warning from remote_actor() to info

This patch downgrades the "remote_actor() called to access a local actor"
warning to an info log entry. This is a followup to #132 and has been an
oversight in the last patch on this issue.
Print OpenCL error log on all systems

This patch removes the "pfn notify" callbacks for printing build errors,
because it is not supported on all platforms.
Make sure brokers stop their backends on exit

This patch makes sure that a broker stops all of its backend when caling
`self->quit()`. Furthermore, this patch uses `cppa::network_error` instead of
`std::ios_base::failure` and adds a new exception `stream_at_eof` to get better
diagnostic in logfiles.
Add missing include to epoll impl
Remove Boost from dependencies

Context-switching actors are no longer supported by the library.
Hence, Boost is no longer an optional dependency.
Fix possible segfault in `peer::dispose`