Scoped all variables to object instance to allow multiple QR Codes

Fixes #2

Previously, private variables were trapped by the outer function
closure that returns the Drawing constructor.

I removed these declarations and scoped them to the object instance.
This has the side affect of making them public.  If that is
undesirable, the code can be refactored further to keep these private.

I haven't fully tested all codepaths (table based drawing, android
specific logic), so more regression should probably be done before
merging this in to your codebase.
Store off reference to this for use with callbacks

In _safeSetDataURI forgot to save off reference to this so callbacks
could keep a reference to the original this.
Revert "Store off reference to this for use with callbacks"

This reverts commit 95b47b9a19afe6e117aab1a26b9416f9713d6150.
Store off reference to this for use with callbacks

Had forgotten to save of reference to this so that callbacks would have
a reference to the original instance.
Merge pull request #3 from qthru/master

It works. Thank you @davidcl64