addresses #74

followed instructions by Christopher Stawarz in
https://open-ephys.atlassian.net/wiki/display/OEW/Plugin+build+files

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.