Add type info to fix runtime deobf of overloaded field names (FML issue 210)

RetroGuard obfuscation can overload field names, i.e., a class can
have the same field name (possibly inherited) and the JVM supports
this field name overloading. Only the name + type descriptor must be
unique.

To deobfuscate these fields correctly, we add the type descriptor
to the mappings. Since the .srg FD mappings do not contain field type
descriptors, they are loaded at runtime.
Enable runtime deobfuscation, finally. Fixes #643.

Now that a fix in the previous commit is in, we can enable obfuscating
the server in the build process, allowing FML runtime deobfuscation
(of both the server and mods) to proceed, allowing obfuscated mods to load.