Implemented MwTimeIsoFormatter in Wikibase

(requires change I0dd02909834ce8b428daea86e8bd2bf87ba9a085)
Implements ValueFormatters extension's TimeIsoFormatter interface in the Wikibase extension
making use of native MediaWiki functionality. Invoking the TimeFormatter is done in I72bd2c4.

Change-Id: I0d36f6a5ec0d7d28deaeed6a71e279b73fd2e449