docs; formatting and tidying

- quick clean up pass on layout and colors for clarity/legibility
- added logo to the sidebar
- fix various css things like a full height page when content is shorter, so the footer doesn't creep away from the bottom
- start using em instead of px for consistency across devices

doc/site/index.markdown

@@ -1,7 +1,9 @@
-^title Welcome
+^title
 
 ## Wren is a small, fast, class-based concurrent scripting language
 
+---
+
 Think Smalltalk in a Lua-sized package with a dash of Erlang and wrapped up in
 a familiar, modern [syntax][].
 
@@ -20,6 +22,7 @@ a familiar, modern [syntax][].
 
     while (!adjectives.isDone) System.print(adjectives.call())
 
+
  *  **Wren is small.** The VM implementation is under [4,000 semicolons][src].
     You can skim the whole thing in an afternoon. It's *small*, but not
     *dense*. It is readable and [lovingly-commented][nan].
@@ -41,6 +44,8 @@ a familiar, modern [syntax][].
     and [an easy-to-use C API][embedding]. It compiles cleanly as C99, C++98
     or anything later.
 
+---
+
 If you like the sound of this, [let's get started][started]. You can even try
 it [in your browser][browser]! Excited? Well, come on and [get
 involved][contribute]!