README.adoc revision

Several changes to improve readme.adoc (started as a minnor typo-fixing). 
- changed ending punctuation. Added . where missing and : where deemed appropiate. Checked with other asciidoctor documentation like the maven-plugin or the reference.
- changed . by - in asciidoctor snapshot to be the same as its real format (that's maybe, beign to picky)
- changed order of titles in blocks according to asciidoc syntax reference. The current configuration works but the syntax reference and other docs use the opposite position. See, 'Code block with title and syntax highlighting' in http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/#source-code
- some typos corrected
I was thinking about that for a while, but in the end this is an asciidoctor example, not HTML. So I decided to add https for two reasons:
- Since it is an example, I think it's better to make thinks as clearly as possible and avoid any ambiguity.
- The generated output includes the import src 'as is' and the js fails to load and blocks the browser for some seconds (at least in chrome).
Imho the examples should always be easy to use and 'idiot proof'.