Add preliminary support for writing 7z archives.



git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1491496 13f79535-47bb-0310-9956-ffa450edef68
merge 7z write support

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/branches/LZMA@1491861 13f79535-47bb-0310-9956-ffa450edef68