Changes to link to design principles of scratch, set limits on new features.
Allow only one HTTP poll at a time per extension

Without this change, long-running poll requests (such as those resulting in a timeout) can cause arbitrarily long request queues, eventually leading to spontaneous termination of the application.
Comment formatting tweak
Fix bitmapResolution in ui.media.MediaInfo
No longer allowing the proxied mouse-down event to continue propagating. For some reason the stageX/Y values changed after I redispatched it.
Add block color editor
Reset scale when restoring block state
Update references nested in other blocks
Merge pull request #299 from LLK/readmeEdits

Changes to link to design principles of scratch, explain about PRs for new features
Fix link in README
Fix build status link in README
Merge pull request #397 from LLK/reset-block-scale

Reset scale when restoring block state
Merge pull request #380 from LLK/fix_sprite_pickup_while_drawing

Fix unintended sprite dragging while drawing
Merge pull request #398 from LLK/update-nested-references

Update references nested in other blocks
Update README.md
Fix layout for show button in small stage mode
Set zoom to 100% after creating a costume from the camera
Conditionally add or skip the extension add button
Now using setSVGData() to properly clear a costume that failed to load.
Fix renaming sounds in nested blocks