Patch for <IE9 browsers

When addtohomescreen.js is used in bundle with other javascript files for web application also supporting <IE9, it will throw an exception due to "addEventListener" not being supported, blocking following code execution. This patch will also prevent useless overhead since those browsers are not mobile capable.
Merge branch 'patch-1' of https://github.com/ejjpi/add-to-homescreen into ejjpi-patch-1
Merge branch 'ejjpi-patch-1'