Remove trailing whitespace
Use real code blocks in README
Replace HTML with Markdown syntax for code blocks
Wrap long lines for better readability
Fix link to ZeroMQ download page

The whole paragraph was rewritten to make the text flow better.
Use emphasis instead of capitals for emphasis
Define all environment variables in one code block

This makes it easier to copy-paste them into a shell.
Fix broken Markdown rendering on GitHub

GitHub is unfortunately using a non-standard Markdown implementation
since they restarted the numbering in the list when there are newlines
before the bullet list or between items in the list.

I believe it's non-standard since I get the expected rendering (with
continious numbering) when I try with newlines at

  http://daringfireball.net/projects/markdown/dingus
Use headings instead of numbered list for steps

I find headings more maintainable: it's now easy to insert/remove
steps in the guide without having to renumber everything in the
Markdown source (to avoid renumbering one could use "0." in the
source, but that looks bad when the source is read as-is).

Using headings also removes a layer of nesting and some GitHub
rendering problems due to the way they interpret numbered lists.
Merge pull request #5 from mgeisler/master

README cleanups