Cygwin setup improvements

Combined the best bits of the improvements suggested by
Ross Smith II <ross@smithii.com> and Joe Fitzgerald <joe.fitzgerald@emc.com>
(via https://github.com/joefitzgerald/packer-windows):

Changelog:

- Removed package dependencies, as Cygwin setup 2.830 no longer requires them to be listed.
- Changed mirror to http://mirrors.kernel.org/sourceware/cygwin/ for speed.
- Replaced hardcoded C: drive references with %SYSTEMDRIVE%.
- Hardcoded architecture to "x86" as "x86_64" seems a bit buggy still, except
  for Windows 2012/2012R2 templates on which 32-bit version seems to crash
  a lot)
- Use Powershell extensions instead of the now deprecated bitsadmin to download remote files
- Changed temporary directory from C:\WINDOWS\temp to %TEMP%.
- Added additional comments
- Split monolithic postinstall script into smaller, more reusable .BAT scripts (which are more appropriate to the Windows platform than Unix-style .sh shell scripts)

Tested these changes, and the boxes

````
win2008r2-datacenter
win2008r2-enterprise
win2008r2-standard
win2008r2-web
win2012-datacenter
win2012-stnadard
win2012r2-datacenter
win2012r2-standard
win7x64-enterprise
win7x64-pro
win7x86-enterprise
win7x86-pro
win81x64-enterprise
win81x64-pro
win8x64-enterprise
win8x86-enterprise
````

successfully install OpenSSH, and VMWare Tools.