Fix Windows drive letter issue(#71)
Fix highlighting candidates problem for Windows
Force absolute option for special case on Windows.

This is work around of GNU global. GNU global does not work well
with not absolute path(root, relative) on Windows if GTAGSLIBPATH
is set. So then helm-gtags.el forces to use absolute path.
Update document for Windows issue
Add test for forcing to use absolute option
Merge pull request #72 from syohex/fix-for-drive-letter

Fix Windows drive letter issue(#71)