Bug when reading manifest from Jar

An UnsupportedOperation exception is thrown when `get_manifest` is called on a
JarInfo object.

This exception occurs because `get_manifest` passes a zipfile.ZipExtFile object
to the parse method of the Manifest class. The ZipExtFile class does not
support `tell` and `seek` operations, and therefore throws the UnsupportedOperation
exception.

To fix this, `get_manifest` reads the zipfile.ZipExtFile object and passes a
string to Manifest's `parse` method.