Added comment support for FormattedSqlChangeLogs

This patch adds comment support for FormattedSqlChangeLogs, similar to <comment> for XML
A unit test is included.

Syntax example:
```sql
--changeset nvoxland:2
--comment: insert some test data
insert into test1 (id, name) values (1, ‘name 1′);
insert into test1 (id, name) values (2, ‘name 2′);
```