WIP: VideoIO with AVOptions v0.01 (compatible with ffmpeg 2.4.2)

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.   



FYI: @assert_type macro indeed works (otherwise I would not have included it), despite the "strange" output from macroexpand. 

```python
julia> @assert_type("")
:default
julia> @assert_type("image_size")
:image_size
julia> @assert_type("pixel_fmt")
:pixel_fmt
julia> @assert_type("video_rate")
:video_rate
julia> @assert_type("video_ra")
ERROR: ArgumentError("option does not exist!")
```