Table Identity Column Check (https://liquibase.jira.com/browse/CONTRIB-14)

Hey Guys,
I've made a patch that should fix the attached JIRA issue. Basically, the SQL server plugin is enabling "identity insert" on tables prior to loading data, but if a table does not have an identity column this blows up. This was preventing me from using liquibase at all in SQL server, as it would not create the DBCHANGELOG table.  

There was a patch on the JIRA ticket that used java-side filtering.  This patch uses T-SQL if statements, which I think is more elegant.

Thanks!
Chris