added doc blocks & code comments
moved set_placeholder() from 'iG_Metabox_Field' to 'iG_Metabox_Text_Field' and 'iG_Metabox_Select_Field' and removed its override from 'iG_Metabox_Radio_Group' because this method is not relevant to all fields
removed set_value(() from 'iG_Metabox_Text_Field' - set_default_value() should be used to set a default value of any input field
updated plugin description & version to 1.0 beta
added set_value() to 'iG_Metabox_Checkbox_Field' class
checkbox should also be checked by default if default value is same as the value of checkbox
moved set_values() from 'iG_Metabox_Field' class into 'iG_Set_Values' trait and imported the trait in 'iG_Metabox_Radio_Group' & 'iG_Metabox_Select_Field' classes
removed set_size() from 'iG_Metabox_Select_Field' - not needed
removed set_maxlength() from 'iG_Metabox_Select_Field' - not needed