Fix merge - duplicate DokanStopCheckThreadInternal dokan-dev#27

Fix x64 prerequisit check

WDK 8.1 Update 1 Only support Win 7 >

Remove readme.txt from installer and add a link instead

Update README.md contribute section

Merge pull request dokan-dev#14 from suy/unmount-bug

Fix DokanUnmount bug

Remove japan readme

Add installer sign script