Fix : Cannot type or search in IE8-9 when shown in a bootstrap modal dialog

This PR fixes #600, #1147 and is simple update of #1051.
Content :
- Add dropdownContainer with the possible values :
-- "body" - default - use body
-- "auto" - closest fixed, then closest absolute, then closest body
-- jquery/dom node - use as is

Has been test on regular page, fixed container (BS modal) and absolute container.
No scrollbar with BS modal case as described in #1051 and spoted in #600
No z-index issue and ESCAPE issue spoted in #1147
No orphan drop-down after close of modal spoted in #600