Support much more DDL in NodeToString.

Including fixes to existing (createTable, dropTable, dropView) and new
(createSchema, alterTable, renameColumn, dropSequence and dropIndex).

Fixes #155
Fix unparsing GROUP_CONCAT
merge nathanlws: Handle more node types in NodeToString

Merge pull request #157 from nathanlws/sql-parser/more-nodetostring-fixes
https://github.com/FoundationDB/sql-parser/pull/157