Polished the build.xml; Updated jar manifest attibutes

[Tjeerd Verhagen <tj...@gmail.com>]

Patch 1: Code / XML Polished (https://issues.apache.org/jira/browse/IVY-554)
I first polished the xml of the build file, created a patch which I added to
the new improvement

Patch 2: Additional descriptions of build.xml targets (
https://issues.apache.org/jira/browse/IVY-499)
When I used the ivy.jar as stand along (not through Ant) I wasn't able to
see the package inforamtion, by using the class Package.
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Package.html
So I moved the following manifest attributes outoff the section specific
part:
- Implementation-Title
- Implementation-Version
- Implementation-Vendor
- Specification-Title
- Specification-Version
- Specification-Vendor
into the main part, but maybe this is not correct in relation to the Ant
tasks (as Ivy is an extention on Ant).
For as far as I know and re-red in the Jar specification
http://java.sun.com/j2se/1.4.2/docs/guide/jar/jar.html the following
attibutes should be

But at least they should then appear in the main manifest attibute part of
the ivy-core.jar.
So I also updated the manifest attributes of the ivy-core.jar creation.

Maybe we should add the following mainfest attribute also:
- Implementation-URL