Include --global PATH; fix main doc link

* hptool/os-extras/win/templates/Nsisfile.nsi.mu
  * Fix the PATH to add the location where binaries
    are installed with "cabal install --global"; this
    was correct in 2013.2.0.0 but got lost in my
    recent shuffling
  * Fix the url shortcut that is installed, to point
    to the platform+ghc master indices, rather than
    just the ghc indices; in moving the HP functionality
    over from 2013.2.0.0, this got munged
  * Fix where a 32-bit HP installed on a 64-bit
    system was going (o, the fun of nsis macros,
    pre-defines, and variables); this functionality
    is new for 2014.2.0.0