remove ABI_VERSION_PROPERTY_NAME which is unused.

Going to do this a different way.
Change ActivatorProperties to export USER_CONFIG_FILE rather than USER_HOME
ActivatorProperties: rename getProperty to getPropertyWithOverrides

this is prep for a no-overrides version
don't lookup user.home in two places
add a getPropertyNoOverrides and use it.

This avoids using "props" directly throughout the file.
Throw exceptions rather than return null on missing ActivatorProperties
remove unused SBT_XMX and SBT_PERMSIZE activator properties
Don't double-namespace env vars for activator.foo props

e.g. to override activator.home you would in theory have to do
ACTIVATOR_ACTIVATOR_HOME without this change, not that anyone
was trying to do that anyway.
Merge pull request #138 from typesafehub/wip/havocp-props

some ActivatorProperties refactorations