Changes the linux platform string, adds IS_LINUX

Part of a change to the way the main SConstruct handles Linux.
Reflects changes to how beast.platform handles Linux

Part of a change to the way Linux is handled.
Merge pull request #1 from evhub/patch-8

Reflects changes to how beast.platform handles Linux
Cleans up code

Also adds info about Arch
Update Platform.py
Merge pull request #7 from evhub/patch-7

Changes how linux is handled in SConstruct