The parsed manifest from a jar file.
Attributes:
main_section: a dict representing the main (first) section of the manifest.
Each key is a string that is an attribute, such as 'Manifest-Version', and
the corresponding value is a string that is the value of the attribute,
such as '1.0'.
sections: a dict representing the other sections of the manifest. Each key
is a string that is the value of the 'Name' attribute for the section,
and the corresponding value is a dict like the main_section one, for the
other attributes.