Implement ability to save Nocturne sessions

[caseif]

Nocturne should implement its own mapping format which saves all data associated with a given session (JAR file, mappings file, members manually marked as deobfuscated) to disk, so it may be easily loaded again and work may be quickly resumed.

This issue will be updated as the feature is further fleshed out.

[jamierocks]

Will this be for 1.1?

[caseif]

1.1 is feature-complete; we already pushed an RC for it.

[caseif]

Still not totally sure I want to do this for 1.2. I'd like to further consider what work it would entail and whether it would be practical to include it in the next release.

[caseif]

Proposal: use an extended version of the JAM format to save session data, using custom tags to store additional metadata and ad hoc mappings.