Updates to the README.
Updates to the README.

Split out the installation instructions to make the platform specifics clearer.
Fixed a bug in the sample code.

dberesford/exiv2node,rbdr/exiv2node,SoftBricks/exiv2node,rbdr/exiv2node,SoftBricks/exiv2node,m0g/exiv2node,dberesford/exiv2node,rbdr/exiv2node,m0g/exiv2node,m0g/exiv2node,SoftBricks/exiv2node,dberesford/exiv2node