downgraded double KILL_PROXY msg warning to info

this actually has no effect on client code, but is merely a minor
libcppa issue, so it souldn't be a warning in the first place;
closes #126
handle invalid_actor in sync send + anon_send_exit

this patch checks in anon_send_exit for invalid_actor and
turns it into a NOP if needed, sync_send throws
std::invalid_argument to have at least a defined behavior
fixed performance issue with prioritizing policy

this patch fixes an issue with the prioritizing policy where the
low-priority cache could slow down the performance of dequeue
operations to the point where the system seems to stop;
this patch also removes the implicit addition of the priority_aware
flag whenever the detached flag is used
fix Windows MinGW64 build
Merge pull request #127 from debio264/windows-fix

fix Windows MinGW64 build
Changed version to 0.9.2
Added VERSION file
Replaced $() syntax with `` in configure script
Enforce coding conventions for namespaces
cleaned up header guards
Updated copyright note
Reorganized configure options

Due to ongoing problems with Boost on Linux distributions, this patch disables
Boost support by default, i.e., turns context-switching off. Users can enable
it explicitly by using `--enable-context-switching`. The old configure options
to disable this feature explicitly are now deprecated. Furthermore,
`--with-opencl` is replaced by `--enable-opencl` and
`--disable-context-switching` is replaced by `--no-context-switching`. This
change implements a more consistent naming scheme. The old option names are
still valid, but give a deprecated warning to the user and will be removed in
the future.
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.