Upgrade to Java 7.

JDK 5 has been EOL'd for a good while. Same with JDK 6.
Allow TarOutputStream to append to an existing tar file. This is
achieved by positioning the file pointer before the EOF-padding bytes
and overwriting them.

Additionally, the existing tests were broken and only executable on
kamranzafar's machine. The tests should now work on any machine.
Wrap the FileOutputStream in a BufferedOutputStream.
Expose bytesRead on TarInputStream by means of getCurrentOffset(). This
can be used to determine where in the tar file a specific entry is
located.
Merge pull request #12 from Nimlhug/master

Allow appending to existing tar files.