Added support for UTC date time labels.

  * dygraph-utils.js:
    new utc switch parameter for `Dygraph.dateString_`
    and new signature for `Dygraph.hhmmss_`.

  * dygraph.js:
    support for UTC date time labels in date formatter.
    Also rename it to `dateAxisLabelFormatter`  for homogeneity
    with the numeric one. and add  a `dateValueFormatter`.

  * dygraph-options-reference.js:
    new `labelsDateUTC` option.
Added support for option `labelsDateUTC` to the date tick picker.

    * dygraph-tickers.js:
      simplified date tick generation. `Dygraph.getDateAxis`
      now produces nice ticks both in UTC and local time.

    * dygraph-utils.js:
      removed `Dygraph.dateSetters` and `Dygraph.setDateSameTZ`
      because they are not needed any more.
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.
Rebase PR 292
Merge remote-tracking branch 'danvk/292-squash' into utc_datetime_labels-issue_498-v2

This is an squash and rebase onto last version of master.
Some minor changes added as per comments in the PR.
Switch to full year in date tick labels.

Use the full year in date tick labels ('Apr 2008'),
instead of only the last two digits ('Apr 08').

Adapt the date ticker test suite for the new label format.
Fix tick location according to local time near DST transitions.

The curent algorithm for tick generation uses the 'roll over'
property of the `Date` methods. Hence, it produces two artifacts:

  1.  A one-hour gap of ticks within an hour of the 'fall back' transition.

  2.  Moves ticks within the hour skipped at the 'spring forward' transition.

To solve the first defect, use the constant spacing in milliseconds
for granularities not coarser than hourly.

To solve the second defect, check that computed dates using the 'roll over'
method return the expected tick position.
Fix comment and revert changes in DST transition test as requested.
Move date accessors to utils script and adapt tick generation and format.

Add two properties to the `Dygraph` object to hold the date accessor methods
according to either UTC or local time.

Use these accessors to simplify a little bit the label format
and the tick generation.
Changed code to separate graph stacking logic by axis.
Fix ghosting issues when viewing graphs with browser zoomed out. Closes #440.
Adding test for multi axis stacking
add date/time axis option to benchmark
Add alias for old method name; revert daylight-savings.html
cleanup
rename labelsDateUTC --> labelsUTC