Several syntax errors not caught

[asmeurer]

I just realized that I never opened an issue about this. After #20, several compile-time syntax errors are warned about, but not all of them. The reason is that when I was going through the CPython source to find the compile-time errors, some of them seemed too esoteric or complicated, so I didn't implement them. A noncomprehensive list of example:

• Invalid use of global and nonlocal variables global/local syntax errors not caught #249
• Some star expression things in Python 3.4. I don't know if pyflakes still supports 3.4, or if any of these are left in 3.5.
• Anything relating to async/await

These probably should be split out into separate issues, but it would also be a good idea to do an audit of compile.c (or just use my original idea, which will catch every compile-time syntax error automatically).