Mixed path separators in 'example/web-app/server' on Windows.

Windows allows either '/' or '\' as path separator though it forbids mixed path separators within one
fs request string. At the same time all URL requests use / in GET/HEAD/etc path.

As result example solutions created mixed path string and forwarded them to fs.open or fs.readdir.

This commit adds convertation of path separators into OS required type.
Fix mixed path separators on Windows platform in tools/bundler.lua

path.join() is the good replacement of hardcoded paths and it provides correct strings for both windows and
posix types of paths.
Merge pull request #417 from norefle/windows-version-fix

Fix mixed path separators in 'example/web-app/server' and 'tools/bundler.lua' on Windows.