run examples and demos with xwalk-launcher

As /usr/bin/xwalk binary doesn't exist anymore, we can not launch
examples and demos from xwalk binary.

In addition, this patch removes tizen-extensions-crosswalk script file
which appears to be useless right now.

BUG=XWALK-2370
Merge pull request #373 from eurogiciel-oss/XWALK-2370

run examples and demos with xwalk-launcher