Update front page blurb

Fixes https://crosswalk-project.org/jira/browse/XWALK-923
Incorporate review comments for front page blurb
Merge pull request #59 from townxelliot/front-page-update-923

Update front page blurb
First draft of new Getting Started

I've split the content over more files (so I could include
both Windows and Linux as hosts, and Tizen and Android as targets).

I removed "Developing on Windows" and incorporated its content
into the new files.
Consolidate all images into one location

There's no reason to maintain images used on documentation pages
inside the wiki, so make a copy of them in the website git repo.
Clean up formatting in Tizen target setup page
Clean up instructions for host setup
Clean up instructions for Android target setup
Improve instructions for Build an Application
Fix links in Run on Android
Clean up Getting Started
Link to Getting Started for usage on Downloads pages

As "installation" doesn't really make sense for Crosswalk Android
any more (the default mode is embedded), remove installation
instructions and just point to the "Getting started" page.
Add instructions to install emulator support rpm

The crosswalk-emulator-support rpm should be installed if using
Crosswalk Tizen on an emulator.
Update instructions for Tizen using SDK 2.2.0
Complete instructions for running on Tizen