Update ClassFile.java

InputStream.read() does not guarantee actually, well, reading anything:

http://docs.oracle.com/javase/6/docs/api/java/io/InputStream.html#read(byte[])

The original code failed for me when reading from a `.jar` file, as InputStream.read() would cut out halfway and leave the rest of the `data` array as zeroes. This cause a NPE in `resolveFromClassFile`:

```
Exception in thread "main" java.lang.NullPointerException
	at soot.coffi.method_info.toName(method_info.java:86)
	at soot.coffi.Util.resolveFromClassFile(Util.java:225)
	at soot.CoffiClassSource.resolve(CoffiClassSource.java:39)
	at soot.coffi.Test$.loadClass(Test.scala:22)
	at soot.coffi.Test$.main(Test.scala:27)
	at soot.coffi.Test.main(Test.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
```

Wrapping it in a DataInputStream fixes that problem on my machine, and should have no affect otherwise
Merge pull request #58 from lihaoyi/patch-1

ClassFile.loadClass is buggy, will only read part of the file