javadocxfer: skip pollution of net.minecraft.server
Document no-chaining option
Skip FML/Forge files added in NMS namespace, to allow remapping MCPC
Assert on invalid argument
Add 13w02b snapshot mappings from FML

Extracted from FML/MCP snapshot15 branch: https://github.com/cpw/FML/commit/f2d42d11344c0b9174ecd1fdea79a276d1092691
Add 13w02b snapshot <-> CB 1.4.6 mappings