Add a changeset to ensure order encounter, orderer and startDate are not null

There is several fields in orders table that have been made required in 1.10, we need a changeset that should be among the first ones if not the first one to run when upgrading to ensure that these fields have been set.

Some code has been written, fetch it from this pull request https://github.com/openmrs/openmrs-core/pull/705

TODO:

• Add a liquibase changeset which checks that there is no order with a null encounter, orderer and startDate, the error message should be very clear to the user about why it failed and how to fix the issue.
  The changeset needs to be the very first one to be run i.e. it should come before 201401101645-TRUNK-4187

Should be something like this:

<changeSet>
        <preConditions onFail="HALT" onFailMessage="Add some descriptive message">
            <sqlCheck expectedResult="0">select count(*) from orders where startDate is NULL or .....</sqlCheck>
        </preConditions>
    </changeSet>