Add preliminary read-only support for the 7z archive format,
together with Copy and LZMA2 decompression,
some tests, documentation, and Javadocs.
Give attribution as per LEGAL-72.

Jira issue key: COMPRESS-54



git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1480055 13f79535-47bb-0310-9956-ffa450edef68