Add API method getAPIs()

This new method will return a String array of all available APIs the server implementation has. So, for example, a CraftBukkit server will only return ["Bukkit"], but a MCPC+ server will return ["Forge","Bukkit"], and Spigot will return ["Spigot","Bukkit"]. I fell this change is needed, not only for the sometime-done-in-the-future Vanilla Mod API, but for better plugin cross compatibility on different server implementations.

The inclusion of this API shouldn't break anything, since it's more of an informational deal for the plugin then anything else. Also, it's recently come to my attention that it's a pain for other server implementations to change the serverName variable without plugins relying on it returning "CraftBukkit" to run properly ([PermissionsEX for one](https://github.com/PEXPlugins/PermissionsEx/blob/7c7a283eb55ece2af71ef73837e2555262a895f0/src/main/java/ru/tehkode/permissions/bukkit/superperms/PermissiblePEX.java#L55)).