Support for unquoted module names in 'import' statements

The compiler will now accept unquoted module names (TK_NAME) as well as
paths/file names (TK_STRING) in 'import' statements.

a2_Load() now adds '.a2s' if no file extension is found in the name
argument.

Also, a2_Load() now takes a flag argument, and a flag A2_NOSHARED has been
added. However, shared modules are not yet implemented, so the A2_NOSHARED
flag has no effect.

Closes #157.