Fixed blendTile()-related blurriness issue

We were setting drawer.updateAgain to the result of each blendTile(),
which meant it was keeping only the last result. Instead we should have
been only setting it to true if blendTile returned true, but never
setting it to false. Fixed.
Don't load the tile source again when creating a navigator.

Previously, when showNavigator was set to true when creating the
viewer, the navigator would unnecessarily load and parse the tile
source, even though a fully parsed object already exists.
Merge pull request #115 from Ventero/navigatortilesource

Don't load the tile source again when creating a navigator.
Merge pull request #112 from openseadragon/blur

Fixed blendTile()-related blurriness issue (for #95)
Removed changelog recommendation from readme as per discussion in #90
Update changelog for recent fixes