Improve docs

Remove whitelines
Update testing docs to reflect changes in library

Add HHVM compatibility as feature

Update TOC

Split Varnish into client and config

Improve docs on proxy client

Clean up Installation chapter

Add flush() to examples so users won't forget to call it

Add proxy config chapter

Finish general introduction

Fix links to introduction

Fix missing ;

Fix grammar

Fix code block

Update cache-invalidator.md

Remove link to self

Fix link to Alternatives

Fix host regex

Add missing Composer install step

Add note on Varnish port

Improve overview

Improve introduction