Option: reorder setters functions

Symmetry with getters.
Option: fix "is of a specific flag" methods
Option: add getters to current value

Also add private functions to refactore check of FFmpeg return value
(raise exception if error).
GeneratorVideo: refactore creation of rgb pixel
VideoFrame: refactore setParameters
Merge pull request #24 from cchampet/fix_get_value_of_options

Add getters to current value of options