Support for multiple portals

[marcopasi]

This would allow to study attack strategies for groups of portals close by (I rarely find myself attacking a single isolated portal). Taking multiple portals into account would be particularly beneficial for XMP bursters with higher range (>L5), resulting in even greater burster optimization.

[xosofox]

This would be a substantial change and quite hard thing to do, but I'll keep it in mind for the 2.0 version ;-)