Optimization work on focal operations.

This commit does a bit of optimization of focal mean. It also implements
"FastFocalMean" a less generic "lower bound" for the kind of performance that
can be expected from a purely sequential focal mean implementation.

It also adds a benchmark to monitor the performance of focal operations.
Update README to reflect 0.7.0 release.
Update focal benchmarks.

This commit adds reference implementations for FastFocalMean (the fastest
single-threaded implementation we could reasonably come up with) and
NaiveFocalMean (a basic, unoptimized strategy).

It also includes updates to Hillshade as well as a bug fix in PNG writing.
Added notes directory for use with herald.
Merge pull request #176 from non/topic/focal-benchmarks

Topic/focal benchmarks
Merge pull request #175 from non/topic/add-herald

Added notes directory for use with herald.
Add new feature and feature set classes, plus initial operations.

This checkin includes the results of our feature spike, which creates
some initial classes for features and featuresets and some sample
operations.  This is still a work in progress.
Add new PNG color modes, and refactor PNG rendering.

This commit adds some new capabilities:

  1. Indexed color, when using <256 colors
  2. Greyscale, when R=G=B for all colors

It also removes much of the legacy PNG writing/rendering code, and unifies some
disparate concepts into a single Renderer class. This should make it easier for
users to get fast, beautiful PNG rendering without having to worry too much
about the underlying implementation.

Some preliminary tests show a ~40% reduction in file size/rendering time when
using indexed color (as opposed to RGBA).
Fix hillshade test to use included test data.
Merge pull request #179 from non/topic/fix-hillshade-test

Fix hillshade test to use included test data.
Fix merge conflicts.
Merge pull request #180 from non/topic/png-indexed-color2

Topic/png indexed color2
Merge remote-tracking branch 'josh/topic/feature_spike'
Re-add RenderPngRgba for compatibility reasons.
Merge pull request #181 from non/topic/render-png-compat

Re-add RenderPngRgba for compatibility reasons.
Improve color handling/allocation.

This commit touches a lot of pieces of the PNG/color rendering process. The
big idea is to try to avoid the situation where we have a mismatch between the
colors requested by a user and the number of classes to be colored.

There are also some changes which are intended to make book-keeping a little
bit simpler. I had wanted to add some demos/examples of the various ways to
"style" a raster but those will have to wait.
Fix test (again).
Add configuration file for Travis CI.
Update link to scaladoc for 0.7.0.
Update travis config to download sbt script.