Fix and enhancements to slime-trace-dialog contrib.

* contrib/slime-trace-dialog.el
(slime-trace-dialog): Change advertised protocol, doesn't fetch
traces automatically, just specs and progress.
(slime-trace-dialog-fetch-specs): New interactive function.
(slime-trace-dialog-fetch-progress): New interactive function.
(slime-trace-dialog-fetch-status): Fetch local control, specs and
progress.
(slime-trace-dialog--clear-local-tree): Simplified.
(slime-trace-dialog--update-progress): "[refresh]" button degates
to `slime-trace-dialog-fetch-progress'.	 "[clear]" button always
shows if there is something to clear. Simplified.
(slime-trace-dialog--on-new-results): Adapt to new
`slime-trace-dialog--update-progress'
(slime-trace-dialog--mode-map): Bind 'g' to
`slime-trace-dialog-fetch-status'.

* contrib/swank-trace-dialog.lisp (dialog-trace): Don't set RETLIST-OF to a
symbol here, backends are responsible for that.
[Fix #72] Convert HACKING to CONTRIBUTING.md
Merge pull request #81 from bbatsov/contributing

Closes #72: Convert HACKING to CONTRIBUTING.md