Allow shared gtest library.
Update ASIO wrapper constructors to only take BoostEnvironment and VMIdentifier

* Restore the behavior of creating a new TCPConnection
  in every call of startAsyncAccept().
Document a few exceptions for the calling thread
Constify a couple members of BoostEnvironment and BoostVM
Improve member order of BoostVM

* The environment is the "main context" of the VM.
* Also follows better the constructor arguments order.
Merge pull request #234 from eregon/boostvm_refs

Update ASIO wrapper constructors
Removed librt linking for OpenBSD
Added compilation instructions for OpenBSD
Fix non-displaying ozc messages in Windows

Fixes #214 : As flag HANDLE_FLAG_INHERIT is used in SetHandleInformation call,
parameter bInheritHandles of CreateProcess must be set to true
Merge pull request #236 from anthonygego/windows

Fix non-displaying ozc messages in Windows
Implement addChildProcess() to kill child processes on termination

* Implements the kill argument of OS.exec.
* Implements the automatic termination of processes spawned by OS.pipe.
* It notably kills the corresponding Tk instance when a VM terminates.
Merge pull request #220 from eregon/child_proc

Implement addChildProcess() to kill child processes on termination
Merge pull request #237 from anthonygego/openbsd

Added support for OpenBSD
Merge pull request #232 from layus/feature/build/gtest_lib

Allow usage of a shared gtest library