Added option for hooks to not have any parameter.

Signed-off-by: Silver Phoenix <silver.phoenix99@gmail.com>
Bindable atrribute's getter and setter now accept a symbol as a method's name.
Removed BindableAttribute#value= and made BindableAttribute#set_value public.
Bindable#set_Value accepts a block that is only executed if the value was changed.
Merge branch 'develop'