Align to Shell Grammar of IEEE 1003.1-2013

This reverts pull request #28 and aligns to Shell Grammar in 2013 edition of IEEE 1003.1 standard whose  `else_part` rule has changed. This pull request also makes parser to accept a single complete command instead of zero or more commands. The latter is intended to help to solve shift/reduce conflicts related to the NEWLINE tokens which in turn is required to proceed with the issue #17. All five shift/reduce conflicts are resolved by removing unnecessary trailing `linebreak` non-terminals from `case_item_ns` rule.