[DT] Add coverage for messages and reduce calls to getClassName

Add some much needed coverage for DataTable messages. Branches not covered are for housekeeping but should never be reached.

Also reduced the number of calls to `getClassName`


```
[14:31][apipkin:~/Projects/yui3/src/datatable(dt-message)]$ yogi test --coverage
yogi [info]  using yogi@0.1.11 on node@0.10.15
yogi [info]  using module: datatable
yogi [info]  prepping grover tests
yogi [info]  adding tests route
yogi [info]  launching xdr server on port 5001
yogi [info]  listening on: http://127.0.0.1:5000
yogi [info]  turning on coverage support in grover
Starting Grover on 12 files with PhantomJS@1.9.2
  Running 15 concurrent tests at a time.
✔ [DataTable: Column Widths]: Passed: 8 Failed: 0 Total: 8 (ignored 0) (0.148 seconds) 86%
✔ [DataTable: Base]: Passed: 17 Failed: 0 Total: 17 (ignored 0) (0.333 seconds) 94%
✔ [DataTable: Core]: Passed: 31 Failed: 0 Total: 34 (ignored 3) (0.457 seconds) 92%
✔ [DataTable: Head]: Passed: 8 Failed: 0 Total: 8 (ignored 0) (0.158 seconds) 98%
✔ [DataTable: Sortable]: Passed: 13 Failed: 0 Total: 13 (ignored 0) (0.261 seconds) 83%
✔ [DataTable: Message]: Passed: 14 Failed: 0 Total: 14 (ignored 0) (0.244 seconds) 100%
✔ [DataTable: Body]: Passed: 49 Failed: 0 Total: 52 (ignored 3) (0.813 seconds) 86%
✔ [DataTable: Table]: Passed: 34 Failed: 0 Total: 35 (ignored 1) (0.524 seconds) 100%
✔ [DataTable: Paginator]: Passed: 11 Failed: 0 Total: 11 (ignored 0) (0.456 seconds) 95%
✔ [Datatable: Scroll]: Passed: 23 Failed: 0 Total: 23 (ignored 0) (1.062 seconds) 77%
✔ [DataTable: Mutable]: Passed: 98 Failed: 0 Total: 98 (ignored 0) (1.253 seconds) 97%
Merge branch 'dev-master' of https://github.com/yui/yui3 into dt-message
✔ [DataTable: highlight]: Passed: 5 Failed: 0 Total: 5 (ignored 0) (6.075 seconds) 100%

---------------------------------+-----------+-----------+-----------+-----------+
File                             |   % Stmts |% Branches |   % Funcs |   % Lines |
---------------------------------+-----------+-----------+-----------+-----------+
   datatable-base/               |     94.44 |     74.29 |       100 |     94.44 |
      datatable-base.js          |     94.44 |     74.29 |       100 |     94.44 |
   datatable-body/               |     87.18 |     73.21 |     93.33 |     86.96 |
      datatable-body.js          |     87.18 |     73.21 |     93.33 |     86.96 |
   datatable-column-widths/      |     86.05 |     71.43 |     88.89 |     86.05 |
      datatable-column-widths.js |     86.05 |     71.43 |     88.89 |     86.05 |
   datatable-core/               |     92.68 |     82.27 |       100 |     92.68 |
      datatable-core.js          |     92.68 |     82.27 |       100 |     92.68 |
   datatable-head/               |     98.98 |     89.29 |       100 |     98.98 |
      datatable-head.js          |     98.98 |     89.29 |       100 |     98.98 |
   datatable-highlight/          |       100 |       100 |       100 |       100 |
      datatable-highlight.js     |       100 |       100 |       100 |       100 |
   datatable-message/            |       100 |     79.31 |       100 |       100 |
      datatable-message.js       |       100 |     79.31 |       100 |       100 |
   datatable-mutable/            |        97 |     88.89 |       100 |        97 |
      datatable-mutable.js       |        97 |     88.89 |       100 |        97 |
   datatable-paginator/          |     95.52 |     88.37 |     97.67 |     95.52 |
      datatable-paginator.js     |     95.52 |     88.37 |     97.67 |     95.52 |
   datatable-scroll/             |     77.78 |     60.76 |     80.39 |     77.78 |
      datatable-scroll.js        |     77.78 |     60.76 |     80.39 |     77.78 |
   datatable-sort/               |      83.8 |     70.39 |      91.3 |      83.8 |
      datatable-sort.js          |      83.8 |     70.39 |      91.3 |      83.8 |
   datatable-table/              |       100 |     84.09 |       100 |       100 |
      datatable-table.js         |       100 |     84.09 |       100 |       100 |
---------------------------------+-----------+-----------+-----------+-----------+
All files                        |      89.9 |     75.99 |     94.14 |     89.88 |
---------------------------------+-----------+-----------+-----------+-----------+


=============================== Coverage summary ===============================
Statements   : 89.9% ( 1380/1535 )
Branches     : 75.99% ( 899/1183 )
Functions    : 94.14% ( 257/273 )
Lines        : 89.88% ( 1376/1531 )
================================================================================
----------------------------------------------------------------
✔ [Total]: Passed: 311 Failed: 0 Total: 318 (ignored 7) (11.784 seconds)
  [Grover Execution Timer] 10.595 seconds
yogi [info]  grover tests complete
```