Started rewriting README

The goal is to begin making this more understandable for people
confused about what FileSaver is and how it can work across all
browsers.
Updated browser support table
Updates to README
Added Chrome for Android to supported browser table
Updated Chrome for Android fields

Removed file size note, since this hasn't beeb tested. Added note to
v28 + on Chrome for Android.
Merge pull request #37 from missing/master

Updated README with supported browser table, to try to further clarify the dependancies and filename support