Added support for getting and setting value of multi select.

Needed to be able to set and get the value of <select multiple> so I''ve added support for ths.
Changes:
- get value now returns an empty array, a single value, or an array of values. No change in behaviour for select-one selects.
- set now accepts a single value or an array of values
- Added tests. All tests are passing
- Incremented version number