Bug: ox-taskjuggler :effort: property incorrectly converted when in minutes

[John Hendy <jw.hendy@gmail.com>]

I ran into an error with a 30min task which used the :effort: property
to set this duration. Upon compilation, I noted the warning:
```
file.tjp:10: Warning: 400.0d of effort of task one does not fit into
the project time frame.
Warning: 1 tasks could not be scheduled
```

I looked at the .tjp file, and 30min had been converted to 4800.0
hours. I was able to reproduce with the following setup:

#+begin_src min-config
  (add-to-list 'load-path "~/.elisp/org.git/contrib/lisp/")
  (require 'ox-taskjuggler)
#+end_src

emacs -Q
M-x load-file ~/path/to/min-config

#+begin_src tj-time-test.org

* Project :taskjuggler_project:
** task1-effort-min
   :PROPERTIES:
   :start:    2014-11-01
   :effort:   30min
   :allocate: jwhendy
   :END:
** task2-duration-min
   :PROPERTIES:
   :duration: 30min
   :allocate: jwhendy
   :blocker:  previous-sibling
   :END:

#+end_src

Exported this via =C-e J j=; report section omitted below.

#+begin_src tj-time-test.tjp

project nil "Project" "1.0" 2014-11-02 +280d {
}
shift s40 "Part time shift" {
  workinghours wed, thu, fri off
}
resource jwhendy "" {
}
task project "Project" {
  task task1_effort_min "task1-effort-min" {
    purge allocate
    allocate jwhendy
    effort 4800.0h
    start 2014-11-01
  }
  task task2_duration_min "task2-duration-min" {
    depends !task1_effort_min
    purge allocate
    allocate jwhendy
    duration 30min
  }
}

#+end_src

Note the effort vs. duration time processing.

I tried the property :Effort: instead, as that's used in Worg and I
wondered if it was "built-in" (even though any tj attribute should be
able to be passed through a property) and got the same result.

From what I can tell, effort/duration/length all have the same
definition structure:
- http://www.taskjuggler.org/tj3/manual/effort.html
- http://www.taskjuggler.org/tj3/manual/duration.html


Thanks for any feedback or requests for more info.


John