Add configuration option to control remapping to future versions. Fixes #913.

This option is most useful on the 151compat builds. If remap-allow-future is
disabled in bukkit.yml, then plugin remapping to 1.5.2 will not be enabled on
1.5.1 (it will be enabled on the 1.5.2-only builds, however), even if
remap-nms-v1_5_R3 and remap-obc-v1_R_3 are enabled.

Fixes plugins which auto-detect the NMS version by checking for class existence
(especially using Class.forName reflection added in previous commit). Before
this commit, they would detect 1.5.2 on 151compat -- and NMS would be remapped
properly, but not OBC since remap-obc-v1_R_3 is not enabled by default. Now,
these plugins (tested with MagicSpells) will detect 1.5.1 on 151compat, just as
other plugins (such as VanishNoPacket) detect 1.5.1 using CB getPackage().getName().
Dynamically lookup shaded OBC version for remapping 1.5.2/1.5.1

Adds getNativeVersion() in PluginClassLoader, replacing the hardcoded
'current' field. This method is used to lookup the obfuscation version
(v1_R_3 on 1.5.2, v1_R_2 on 151compat, etc.), and now gets it from the
actual version package in the server jar, rather than hardcoding. This
means it is correct on both 1.5.2 and 151compat builds, where previously
the version was v1_5_R2 in both cases.