Initial support for multicore seeding
Added URI support to WMS driver as per: https://github.com/gwaldron/osgearth/issues/345.
Merge pull request #346 from ebabyak/master

Added URI support to WMS driver as per: https://github.com/gwaldron/osge...
Slight tweak to ReaderWriterWMS to try reading the image first instead of the string.

Hopefully most of the time the request will be a success so it will
avoid a double hit on the server.
Merge remote-tracking branch 'origin/topic-multicoreseed'
Added documentation for new --threads and --estimate options.

Fixed warning message from osgDB
Merge branch 'upstream/master'