web: UI improvements for zero-length spans

Two minor improvements for spans with length zero:
* Draw span bars with small width instead of zero
* Show bar labels even if the span is of length zero

Author: @franklinhu
Fixes #299
URL: https://github.com/twitter/zipkin/pull/299
Remove Google Analytics
Sort spans by duration after start time.
Merge pull request #302 from elliottneilclark/sort_duration

Sort spans by duration after start time.
fix split node group
Merge pull request #303 from fulinlin924/master

fix split grouped nodes
Introduced Cache-Control HTTP headers for resources.
Merge remote-tracking branch 'upstream/master'