Ensure absolute path is used for library

[`System#load`][1] requires an absolute path.  If the extraction
directory (`library.${name}.path` or `java.io.tmpdir`) is relative, the
extraction will succeed, but the `System#load` call will fail, resulting
in the library not being loaded.  This commit rectifies that by
obtaining the absolute path to the directory before extracting.

[1]: http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#load(java.lang.String)

hawtjni-runtime/src/main/java/org/fusesource/hawtjni/runtime/Library.java

@@ -262,6 +262,9 @@ private String map(String libName) {
 
     private File extract(ArrayList<String> errors, URL source, String prefix, String suffix, File directory) {
         File target = null;
+        if (directory != null) {
+          directory = directory.getAbsoluteFile();
+        }
         try {
             FileOutputStream os = null;
             InputStream is = null;