File reader support

Shapefile and DBF constructors now support referencing files
via both url and a HTML 5 File API handle. This can be used
to load and map content added to file input(s) and processed
on the client browser.

FileReader and FileReaderSync are both used depending upon
availability to make this functionality work in both Gecko
and Webkit inside Web Workers.

This pull request includes merges of my previously submitted
pull requests including fixes for missing dbf files and Point 
features.