Improve SQL introductory text.
Fix formatting, improve structure, add exercises in SQL section.
Add discussion of null and inconsistent data.
Change license to CC-BY (from CC-BY-SA) to be compatible with upstream Swcarpentry.
(Needed if we want to contribute back.)
(Also, add list of contributors for acknowledgements.)
Rename examples.md to solutions.md.
Fix formatting of solutions.
Bring solutions into sync with exercises.
Merge pull request #12 from benwaugh/master

Update SQL material
Move section on nulls earlier in the lesson.
Add section on subqueries.
Move SQL views to list of advanced topics.
Create table and insert row.
UCL adaptation of testing materials
Merge branch 'master' of github.com:UCL/ucl_software_carpentry
Fix formatting in SQL code.
Fix error in SQL query.
Acknowledgments for SQL material.
Remove duplicate/unused SQL exercise solutions.
Add text about SQLite.
Specify CC-BY licence for SQL material.