Make the metadata cache time a static so it can be overruled.

Especially usefull in situations where you not only make the current version of a plugin available, but also older (versioned) versions. For the older versions, you can presume that the meta data will not change anymore and can choose to use a longer cache time.
Merge pull request #12 from jrfnl/Filter-metadata-cachetime

Make the metadata cache time a static so it can be overruled.