Add support for JavaScriptCore.

[larsbrinkhoff]

Seems to work quite well, except for one thing: readline doesn't seem to detect EOF when reading from a pipe. So the tests just hang when they reach the end of the input file.

[pipcet]

Hmm. There appears to be no way to distinguish EOF from an empty line in jsc.cpp:

2336    EncodedJSValue JSC_HOST_CALL functionReadline(ExecState* exec)
2337    {
2338        Vector<char, 256> line;
2339        int c;
2340        while ((c = getchar()) != EOF) {
2341            // FIXME: Should we also break on \r?
2342            if (c == '\n')
2343                break;
2344            line.append(c);
2345        }
2346        line.append('\0');
2347        return JSValue::encode(jsString(exec, line.data()));
2348    }

[larsbrinkhoff]

Merged without the tests.