Adding support for Minecraft 1.6

This pull request should add basic support for Minecraft 1.6

It does the following:
- Adds support to parse the Minecraft JSON blobs that specify library dependencies (with tests)
- Parses the JSON included with FML to add build dependencies and download natives to jars/bin/natives
- Fixes fixing the *.exc file (Contained some invalid java signatures in 1.6, it now skips processing those and writes them to the output mapping unchanged)
- Downloads the Minecraft client using the new URL format (URLs are no longer specified in FML config)

Current issues I know of:
- When switching MC versions, one has to do a manual removal of the build directory (clean doesnt seem to pick it up), since the extracted deobfuscated jar is still there and gets picked up by the decompiler. This causes compilation issues because MinecraftApplet and other classes are gone in 1.6
- Didn't test Eclipse integration (since i don't have it installed).

This should be considered a starting point however :-)