Vicious Little Interceptors and Difficulty Levels

Added in some nasty little Interceptors for a bit of variety. They are more aggressive on the way in but fire less on the way out.

Difficulty level code is implemented, at least in the gameplay - still need to add the menu options. At the moment it ranges from Space Slug through to Impossible.

A couple of screens showing me getting into a bit of bother:

I have uploaded a video of me playing on Insane (second to hardest):

http://www.youtube.com/watch?v=NE5O2rlFRM8&feature=youtube_gdata

At the moment, I can't beat it without losing at least one life. Need more practice!

The final game will have the option of an energy bar (shields) and for the truly dedicated arcade headcase a 'Hardcore Mode' with single shot instadeath.

Comments welcome.