lib/Geo/GoogleMaps/OffsetCenter.pm: Bugfix

Math::Trig is necessary for pi.
lib/Geo/GoogleMaps/OffsetCenter.pm: Rename old method

We want to be able to offset by multiple ways, so I'm adding the input
technique into the name of the call, to make it less confusing.
lib/Geo/GoogleMaps/OffsetCenter.pm: offset_center_by_pixel

Now, you can offset the center by pixels.

You have to specify:

    * width and height of your image
    * x/y coordinate of where in your image you want the lat/long to appear

It is origin'ed on the bottom left, so it's easy to visualize as simple
cartesian coordinates.
lib/Geo/GoogleMaps/OffsetCenter.pm: POD fixup

Arranging things a little nicely.