Inherit $_ENV by default in AssetProcess

Windows needs the SystemDrive and SystemRoot environment variables to run
node.exe. Seemed reasonable to inherit the whole Environment by default
for proc_open. Variables passed to AssetProces::environment() will alway
override the ones set in global $_ENV.
Wrap command in quotes on Windows

If the command to AssetProcess is something like C:\Program
Files\nodejs\node.exe, it will be wrapped in quotes "C:\Program
Files\nodejs\node.exe". This improves out of box experience for poor
Windows users using the default install location of node.js. Before you
had to double-quote the node variable in asset_compress.ini like:

node '"C:\Program Files\nodejs\node.exe"'

Now you can just write:

node C:\Program Files\nodejs\node.exe

This has no effect on Unix commands like /usr/bin/node
Bug fixes to Windows node.js

The regex was capturing the space after executable name but not
replacing it which meant parameters glommed on to the exe name.
The $_ENV superglobal is not normally set in production environments
so we have to select the environment variables to inherit by name.
Code reduction

I did not realize you could cast a string to an array so simply.
Merge pull request #183 from planardothum/min-for-win

Support node.js on Windows.

Fixes #182