Pl3xmap support and Map class abstraction

[lexiccn]

Closes #25

Important Notes about specific sections of this PR

• Pl3xMap version check (TownyProvinces#loadIntegrations) is made the same way as MapTowny, i.e using the same class checks
• Dynmap Icon registration (DisplayProvincesOnDynmapAction constructor) is made the same way as MapTowny also as they similarly ran into the need to use a BufferedImage for a Dynmap icon, so needed the same conversion
• Finally, layer creation (DisplayProvincesOnPl3xMapV3Action#createLayer) is similar to the MapTowny method, this one is less intentional but worth noting as I did use MapTowny here to better understand Pl3xMap api where documentation was a bit rough (i.e the registries)

I did not think these justified inclusion of MIT licence as they are not "substantial portions" nor even directly copied code, I just used MapTowny as an example of Pl3xMap support and felt this should be mentioned to make it clear I was aware of MapTowny code, similarity is not coincidental

I am happy to update to include MapTowny licence for these parts otherwise, but I figured this was acceptable use particularly as it is also under TownyAdvanced org and not copied word for word, only the concepts behind these parts

Besides these concerns, this PR adds:

• Ability to set a custom Town Costs Icon (by default, Dynmap is set to not use this)
• Ability to use Pl3xMap v3 with TownyProvinces
• Ability to use multiple mappers at once with TownyProvinces (e.g Dynmap + Dynmap-Towny & Pl3xMap + MapTowny can be run simultaneously with this to some nice results if so desired)

[Goosius1]

• FYI I'll be waiting for Llama to get back to sort out some licencing issues before merging, so don't worry if there's a delay in merging, that's all it is.

[Goosius1]

• Fyi I clarified the licencing with Llama. Currently TP is just the normal/default setup of no licence / all rights reserved, so since this PR did not copy any code of other plugins, that pre-existing setup won't change with this PR, although I will think about that stuff more after all the major fixes/requests have been done in the next few weeks.
• Now that that's clarified, I'll continue this review and see about merging.