Add caching to slider plus setting

[Manoj Solanki]

Add Moodle caching. Also add a setting allowing it to be configurable with a default of 5 minutes.

[Manoj Solanki]

Fix #11. Requires further testing.

→ <<cset cdf8b8d4fbcc>>

[Manoj Solanki]

Fix #11. Fix issue following testing to ensure cache is rebuilt once cache expiry time really expires. Previously last build time was based on last user access so this change will ensure they don't miss announcements unnecessarily.

→ <<cset 8d337f40833a>>

[Manoj Solanki]

Fix #11. Updated to handle the non-existance of lastcachebuildtime array parameter during first run of caching (usually when first installing, or updating slider to a version that includes caching functionality.

→ <<cset eb8b18980625>>