Add external links to choices fields

You can now use the "clickURL" key in a choices field definition to
generate an info button to provide more information.

A sample field definition is below:

```xml
<dict>
  <key>class</key>
  <string>OTMChoicesDetailCellRenderer</string>
  <key>fname</key>
  <string>pests</string>
  <key>clickURL</key>
  <string>http://www.forestry.gov.uk/pestsanddiseases</string>
  <key>key</key>
  <string>tree.pests</string>
  <key>label</key>
  <string>Pests</string>
  <key>minimumToEdit</key>
  <integer>0</integer>
</dict>
```