Allow user-level activator configuration

This is explicitly so that we can configure the http proxy is ONE file (the same location) for Windows/Mac/Linux, so our instructions are simpler.   Also, this configuration file works even with copying the bat/bash scripts into project directories.  Once configured for a user, any activator will use these JVM settings.

Amazingly, cygwin has this fun feature where it assigns you a new user directory, but the existing windows properties still exist so Java just ignores it.   We hack our way to parity by also ignoring cygwin's notion of a home directory.

Review by @havocp.