Add detection for Windows 8.1

This adds a new token for Windows 8.1 (win8_1) as well as a new token
for Windows 8.0 (win8_0) so that they can be detected independently
but the general token (win8) can be used for both.
Merge pull request #69 from dougwilson/feature/win81

Add detection for Windows 8.1
Bumps version to 1.63.
v1.63

    - Adds detection for Internet Explorer 11 (Douglas Christopher Wilson)
    - Adds detection for Windows 8.1 (Douglas Christopher Wilson)