(SERVER-167) Add documentation on `set_trace_func`

This commit adds some documentation on the use of the Ruby
`set_trace_func` function in the JRuby environment to trace events in
Ruby code when running in Puppet Server.
(SERVER-167) Added language to syntax highlights

This commit adds a language to a few of the syntax highlights in the
`dev_trace_func.markdown` page.
Merge pull request #279 from camlow325/maint/master/SERVER-167-document-trace-func

(SERVER-167) Add documentation on `set_trace_func`