Option: fix get type of AVOption

An AVOption corresponds to a choice if it has a unit and a type of
AV_OPT_TYPE_INT or AV_OPT_TYPE_INT64.
OptionLoader: warning if found a child option with no parent option
Merge pull request #81 from cchampet/dev_option_fix_unit

OptionLoader: fix AVOption which correspond to a choice parameter