Added request header support to HTTPClient
Reworked most of the methods in HTTPClient to just use the HTTPRequest object instead of a string.
Added setLastModified function to HTTPRequest to add the If-Modified-Since header
Aded RESULT_NOT_MODIFED to the list of codes in ReadResult
Merge branch 'master' into topic-lastmodified
Updated calls to use the last modified date from the cache.
Merge branch 'master' into topic-lastmodified
Making the file system cache always return the last modified time of entries
Removed the expiration logic from CacheBin in favor of allowing the caller to decide what to do the date.
This means that cached data will always be returned even if it expires.
Changed ImageLayer and ElevationLayer to check for expired cached entries and try the TileSource, but if the TileSource fails for any reason the cached data can still be returned.
Expiration logic in URI
Removed calls to CachePolicy::NO_CACHE.apply throughout
Removed commented out code
Writing all HTTP headers to the metadata.  We can use these for doing proper HTTP cache control or etag support at some point.
Removed debug code
Corrected capitalization of Content-Type header
Removed NOT_MODIFIED from the list of recoverable codes
Updated LevelDB to use the changed CacheBin interface.
Always making LevelDB return the last modified time
Added readymap-tilecache.earth example
Added new color ramp driver.
Added ramp option
Added colorramp.earth example and some sample color ramps