Added Xcode project for OpenEphysHDF5Lib, which builds a shared library (libOpenEphysHDF5.dylib) and installs it in open-ephys.app/Contents/Frameworks.  On macOS, the KWIKFormat plugin now links against libOpenEphysHDF5.dylib.

addresses #74

Set MACOSX_DEPLOYMENT_TARGET (i.e. the minimum supported OS X version) to 10.9.  Pass -Wpartial-availability to the compiler so it will warn about usage of API's that aren't available on all supported OS X versions.  Removed /opt/local and /usr/local from OS X header and library search paths, as they're used only by specific plugins and not needed generally.

Created Xcode projects for all currently-built plugins.  Added Xcode workspace with "All" scheme for building GUI and all plugins.  Addressed some compiler warnings in plugin code.