I've seen several issues where users followed platform-specific install docs, but were unaware of additional prerequisites spelled out in the top-level INSTALL.md. So let's try making each platform's installation documentation self contained.
Rewrote some copy to provide more context and link to more GL-related repos, including GL JS. Put the various SDKs in a table so we can show the various CI bot status images.
Integrated target list into the relevant documents. Rewrote large portions of iOS and OS X setup documentation to guide normal developers to more friendly places. Combined some Android documentation.
Moved SDK-specific documentation into platform/. Added a temporary readme in ios/ that points to the one in platform/ios/.
