Null Parameters
Added tests to test whether `null` value parameters and clause parameters where being handled correctly:
- `INSERT` tested with multiple value parameters (including nulls)
- `UPDATE` tested with mutliple value paremeters, null and non null clause parameters.
- `SELECT` tested with null clause parameters.
Altered SqlAbstract to build the parameter list correctly with null values.
Altered the data statement to store null parameters instead of filtering them.
Merge pull request #33 from russholio/null-parameters

Null Parameters
Distinct
Allow for DISTINCT values to be selected in queries.
Merge pull request #34 from russholio/distinct

Distinct