Add an option to emulate US layout

Under some layouts (e.g. ru and cz), ASCII keysyms are not so
accessible as US layout.  In that case, people want to switch to the
US layout automatically, while other people don't want that switch.

This patch adds a user option "use_us_layout" to emulate US layout
when certain engine is active.

BUG=https://bugzilla.redhat.com/show_bug.cgi?id=760108

Review URL: https://codereview.appspot.com/7368043