Fix Plist parsing of binary blobs

This handles binary blobs by interpreting their data as an NSString (ultimately resulting in a std::string after parsing is completed). It leaves it up to the user of the API to determine what to do with the bytes stored in the resulting string.
Can you key the path off of the current path of the binary? For example, since this builds into `build/darwin/osquery/filesystem/darwin` can you put the plist in `tools/` and do like `$CURRENT_PATH/../../../../tools/test_binary.plist` or something.

Also, please commit the test plist.