Update README
ImageMap now accounts for screen density factor. Until now it always used density 1.0,
which corresponds to Android MDPI, so areas and touch handling were done incorrectly on HDPI or higher resolutions.

Instead of using HashMap<Integer, Area> we now use SparseArray<Area> for a slight performance improvement.

When creating an ImageMap, new parameters can be defined in XML:
- ctc:fitImageToScreen
- ctc:mScaleFromOriginal
- ctc:maxSizeFactor (how big the pic scale factor can be; defaults to 1.5f)

The OnImageMapClickedHandler interface has been changed:
the onImageMapClicked() callback now accepts an area id as well as the ImageMap itself.
This has been done for the case of searching for something across several imagemaps.
Refactored so that the default value for mMaxSize is a separate final variable.
Added Android studio instructions to README
Minor README reformatting
Merge pull request #6 from fess89/master

Accounting for screen density and extra XML attributes