Support for using NULL value in update selection criteria

This patch add support for allowing NULL to be used as row selection criteria. The current version of DbFit fails with something like this:

     | Update | sometable |
     | col_a | col_b= |
     | NULL | 42 |

because the update statement generated compares the values using '=', which doesn't work for NULL. My version handles this correctly.

I have modified all of the _UpdateTest_ acceptance tests to include this case (sorry if I've mucked up the line endings for the _SqlServerTest_ cases and some file permissions seem to have changed too). There are unit tests covering the modifications to the _dbfit.util.DbParameterAccessor_ class, which was modified to allow binding a single input parameter to more than one position in the prepared statement.

Neil