Update URLs pointing to download.01.org.

There has been a big reorganization as discussed in 
https://lists.crosswalk-project.org/pipermail/crosswalk-dev/2014-March/001215.html. 
In summary, a URL such as

  /releases/android-x86/canary/crosswalk-5.6.7.8-x86.zip

is now hosted at

  /releases/crosswalk/android/canary/5.6.7.8/x86/crosswalk-5.6.7.8-x86.zip

Similarly, Tizen RPMs have undergone the following change:

  /releases/tizen-mobile/canary/crosswalk-5.6.7.8-0-i686.rpm

is now

  /releases/crosswalk/tizen-mobile/canary/5.6.7.8/crosswalk-5.6.7.8-0-i686.rpm

Additionally, the Android URLs are going to change again starting from the 
next canary and next beta -- the main zip files are going to be 
architecture-independent, like this:

  /releases/crosswalk/android/canary/5.6.7.9/crosswalk-5.6.7.9.zip

This adjustment has not been done because these new binaries have not been 
produced yet, though.

The new layout also allows us to simplify getXwalkDownloadUrl() by making the
|version| argument mandatory, since each binary is in a separate directory
now.