JumpMap and Credits tweaks

Three slight tweaks:

• Further cleaned up coordinates documentation in the API docs
• While doing that, added support for sector/hex and sx/sy/hx/hy in addition to x/y for specifying the location for the Credits API.
• After bludgeoning at GDI+ with a pointy stick, made the JumpMap API spit out transparent PNGs, so the bit outside the border will blend more appropriately with hosting sites. Sorry, transparent GIFs are too much work.

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