RFR: Javadoc and sources

[Jiří Vaněk]

To proceed with release to maven repositories, source and javadoc jars are mandatory. Thus adding the,

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Commit messages:
 - Added generation of maven source jar
 - Added maven javadoc generation

Changes: https://git.openjdk.java.net/asmtools/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=asmtools&pr=19&range=00
  Stats: 34 lines in 2 files changed: 33 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/asmtools/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/asmtools pull/19/head:pull/19

PR: https://git.openjdk.java.net/asmtools/pull/19