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.   



I already found a while ago that `get_camera_devices()` did not return anything on my OSX 10.9.5 installation of VideoIO (both v0.3.1 and v0.4-dev with master).  If I remember correctly others also reported that VideoIO could not always auto-detect cameras both on Windows and OSX.  This was the very reason for trying to set up the DeviceCapabilities API.  I am definitely using `master` now with v0.4-dev, and in any case, I have not changed `get_camera_devices()`.   I always had to manually set the DEFAULT_CAMERA_DEVICE to "Built-in-iSight" at least on my computer.  What I saw is that `open(`$ffmpeg -list_devices true -f $idev -i $idev_name`) failed to return anything and m was empty. 

Here is the output I get when I run get_camera_devices:
```python
julia> CAMERA_DEVICES = get_camera_devices(ffmpeg, "avfoundation", "\"\"")
0-element Array{UTF8String,1}
julia> CAMERA_DEVICES = get_camera_devices(ffmpeg, "qtkit", "\"\"")
0-element Array{UTF8String,1}
```

When you run the same on OSX, do you get a filled CAMERA_DEVICES?  I am not sure what is going on, but this never worked for me - even before I started any work on VideoIO.