WIP: VideoIO/ffmpeg2.3 with AVOptions v0.01

AVOptions API
============
Maximiliano Suster
October 2014

AVOptions provides a generic system to get/set options for Devices, formats and Codecs.
It relies on AVOptions, AVDictionary and AVDeviceCapabilitiesQuery APIs.
AVOptions API is contained in src/avoptions.jl.

Currently the following functions in work:

       document_all_options
       print_options
       is_option
       set_default_options   
       set_option 
       get_option
       create_dictionary
       set_options_with_dictionary

Ongoing work:
The following functions depend on the DeviceCapabilitiesQuery API.  Some require the newly updated ffmpeg 2.4 libraries (>=AVUtil 53). 

      query_device_ranges,
      set_device_with_query,
      list_devices
      get_metadata 
      
Examples and documentation will be presented once completed.   



Couple of issues here: 
1. You create `dictionary` and store a pointer to the object in `pDictionary`.  However, that doesn't store a reference to the dictionary itself anywhere (the pointer doesn't count), so it will be garbage collected at some point in the future.  If you need to keep it, you need to store it more permanently (e.g., inside an object of some type that you return to the user).
2. Number 1 doesn't actually become a problem, because you immediately overwrite the pointer to `dictionary` with `C_NULL`.  

If this was done on purpose, then the previous three lines aren't needed...