Add container option (#83, #37)

Added the `container` option/data-attribute. When enabled, the bootstrap-select is appended to a specific element and absolutely positioned in the same spot as the original select, e.g. `container: 'body'`. This should solve the problems people were having with the menu being cut off when the select was located in a container that didn't have `overflow: visible`.

Set `container` globally, or simply add `data-container` to the select, e.g. `data-container=".main"`.