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