Updated premake file

Made a few changes to the premake file so C# and C++ project mix better,
including a spiffy new icon for the C# libraries.
This also has initial support for building the engine for Xbox360 and
PS3. Currently Xbox360 will compile, but PS3 has some problems.
(For future reference; while console support is available I have limited
opportunity to test and fix this so it won't be uncommon for these to be
broken. I'll try to limit this to branches though.)