Improve DynamicScene path visualization

Previously, we were only using data points explicitly specified in the CZML to draw paths.  This change makes the sampling algorithm take a maximum step size of 60 seconds so that if points are further apart, we step in-between them.  This allows us to generate certain types of data (such as satellites) with a greter step size in the CZML.  For example, I regenerated simple.czml and it is now 50% smaller with no visual differences.

While the path code is still not in its final form, this change provides a big benefit from a minor tweak.  The easiest way to test this is to use the new simple.czml in master and notice how jagged the orbits are, loading in this branch shows smooth orbits.
Add `Color.toBytes` to match `Color.fromBytes` (and also to make tests more readable)
Add `Texture.clear`.

Also fill in more documentation, and clean up the code and the specs for Texture.
Fix logo processing.

* make logoOffset work properly
* clear texture before copying in new logo regions
Remove `Texture.clear` and instead just delete and recreate the texture every time it changes.

texImage2D doesn't work reliably with a null source, and apparently I'm crazy for wanting to clear a texture anyway.
Move sampler-related specs to a separate spec.
Merge pull request #503 from AnalyticalGraphicsInc/sampledPaths

Improve DynamicScene path visualization
Adjust default camera position and make consistent.

Previously the widgets adjusted the default camera position after constructing the scene, meaning that users had inconsistent defaults depending on whether they used a widget or not.
Merge pull request #508 from AnalyticalGraphicsInc/camera-default-position

Adjust default camera position and make consistent.
Fix an off by one error in CZML handling

When CZML packets have overlapping data within the same interval, we were improperly merging them together, resulting in out of order data.  I fixed the issue and added a unit test to cover it.
Merge pull request #509 from AnalyticalGraphicsInc/offByOne

Fix an off by one error in CZML handling
Merge pull request #507 from AnalyticalGraphicsInc/logo-offset

Fix problems with logos
Merge remote-tracking branch 'origin/master' into models