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.   



Thanks for the suggestion.  I  would  even shrink to the following:

```python
function assert_type(ex::String)
    if !(ex=="" || ex=="image_size" || ex=="pixel_fmt" || ex=="video_rate")
     throw(ArgumentError("option does not exist!"))
   end
end
```
Then we call `assert_type(OPTION_TYPE)` in `get_option()` instead of a macro that returns the same input string.