Add explicit support for history in VRT files

[neilflood]

This addresses the discussion in #10 around VRT support

In the end, the solution was even simpler than the one discussed.

• It is an exception to give parents when adding history to a VRT file
• Only currentfile metadata is saved in the VRT. No lineage information is stored at all, deferred or otherwise.
• When reading history out of a VRT, all component files must be present (or the VRT would not work). So, the read function recursively reads the history from component files and adds it as though they were parents of the VRT. This creates the lineage on-the-fly.
• The code for merging in parent history has moved to be a method in the class, so it can be shared between normal creation of a history object and on-the-fly creation for a VRT.

Note that this solution still does nothing to address the metadata size overflow problem mentioned. This mainly relates to GTiff format, but still needs care. It is possible we should do some separate fix for this, but that can come later.

[gillins]

Looks really good and very neat!