rewrite VelocityTracker to work with varying frequency samples

- improve fling behaviour
- should fix #49
serious refactor: TileLoader:

- no more duplication of TileLoaders for GWT ->
- decouple loadTile() from TileDataSource completed() call
- all TileDataSource MUST call completed(success) in any case now
fix: ElementLayers, make sure setFrom() clears previous items
refactor: extract theme RulerBuilder from Rule
add select="when-matched" rule option
rename RenderThemeHandler - XmlThemeBuilder
cleanup: RenderTheme
cleanup: XmlThemeBuilder:

- rename from -> read
- cleanup theme attribute validation
add TextBuilder for Text styles
app++
start ThemeBuilder api
cleanups: theme/rule:

- rename destroy -> dispose
- use EMPTY_RULES array to avoid null checks
- squash: RuleVisitor
start Line and Area builders
use AreaBuilder in XmlThemeBuilder:

- docs
- better Area field naming
update LineBuilder
rename theme style classes
remove LineSymbol
theming changes:

- select="first" also add its renderstyles when no sub-rule matched
  ... before they where only added when one subrule matched

- use select="when-matched" as last subrule in select="first" rules
  instead.
Merge branch 'theme_refactor'
cleanup: move carto theme to not working mapnik tilesource