Autolinking

* No explicit `clay -lsmth` needed to make module work:

```
clay/test/lang/modules/imports/library$ cat main.clay
import "curl";
external curl_easy_init(): OpaquePointer;
main() {
    curl_easy_init();
}
clay/test/lang/modules/imports/library$ clay main.clay
clay/test/lang/modules/imports/library$ 
```

Found the idea in [Doug Gregor's presentation](http://llvm.org/devmtg/2012-11/Gregor-Modules.pdf) on slide 67

* Second commit fixes a bug on my system after upgrade: llvm-jit-linker needs `smth.so` instead of `smth` without an extension. I tried to revert to the first patch where `linkLibraries` appeared, but it has the same error. It didn't have, I don't know why. Please test something like `clay -lsmth foo.clay`, if it is only my system's trouble or not.