Fix client architecture algorithm for Win64

Prior to this change, the algorithm would try to guess the client
architecture by checking for the presence of "Win64" in the platform
string. Resources on the web
(https://stackoverflow.com/questions/19877924/what-is-the-list-of-possible-values-for-navigator-platform-as-of-today)
argue that this string is not always advertised even on 64-bit platforms.
So we're giving here 32-bit binaries to 64-bit clients...

This commit fixes that algorithm by also checking for "WOW64"
in the user agent string. This is the proof that the Windows
client has the WoW64 subsystem, present on 64-bit Windows.
Merge pull request #452 from bclozel/sagan-452

Wrong arch guessed in tools downloads