Format year of monthly tick lables with 2 digits instead of 4.

Since commit 7b2dfd0 (Oct 15, 2013) the format of monthly tick
labels is `%b %Y` (e.g. Oct 2013). Before the format was '%b %y'
(e.g. Oct 13). Revert to the later format because it seems to be
an accidental change.

  * `dygraph.js`:
    Revert format of monthly tick labels from `%b %Y` to `%b %y`.
Insignificant fix of `>` in jsdoc comments.
Added UTC date label tests and adapted existing ones.

    * tests/labelsDateUTC.html:
      New test/demo for the option  `labelsDateUTC`.
      Display several days of data according to UTC and to local time,
      and compare the tick placement and the label format.

    * tests/x-axis-formatter.html:
      Added `opts` argument to `axisLabelFormatter` function
      (used by inner `Dygraphs.dateAxisLabelFormatter` call).
Added automated test for UTC date ticks and labels.

The date tick tests are broken in the ticker automatic test suite
because they assume EST timezone. In addition, the suite is disabled.
Fix the  automatic test suite for the case UTC ticks and labels:

  * `auto_tests/tests/date_ticker.js`:
    Test the generation of ticks and labels at nice time positions
    according to UTC as set by option `labelsDateUTC`.
    Include the date axis tests from `auto-tests/tests/tickers.js-disabled`
    with the required changes of timestamps and with initial ticks
    out of range removed.
Removed disabled ticker test suite (no longer needed).

The tickers test suite is disabled. In addition, most tests there are broken.
Date ticker tests are fixed and moved to a separated date ticker test suite
by last commit. Hence move numeric ticker tests to the numeric ticker
test-suite and remove the disabled tickers test-suite.

  * `auto_tests/tests/numeric_ticker.js`:
    Include tests from `auto_tests/tests/tickers.disabled-js`
    but commented because they are all broken.

  * auto_tests/tests/tickers.disabled-js:
    Delete it because it is no longer needed.