Map +Oil Rigs version. Corrected access to bridges for repairs and terra...

Poland Raid v4 changes:
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+ access to bridges from south has been corrected for engineers
+ added Oil Rigs with custom cash-trickler
+ some terrain modifications to improve balance for starting zones
  - graphical comparsion is avalible at http://content.open-ra.org/?p=maps
+ ore fields is now more balanced
+ ore values are slighty reduced for map needs
+ starting cash is set to 4000 and starting units restricted to MCV only
  - the last rule is to give spotters bigger chance to go opposite shore before enemy defences will be too strong - and to delay optional destruction of bridges

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*Please Be cautious with non-naval AI bots, while bridges can be destroyed. It may slow your games until route alghoritms for AI bots will be improved.