Formatting fixes, XSD updates, CSS fix for code examples, formatted SQL preconditions

- Fix CSS issue where code/XML example text would escape from its div in Firefox 26.0. (Chrome just wraps the text.)
- Fix properties example in Maven index.md
- spring.md - sort the sub-tags in alphabetical order.
- Add info to README.md, on building the site locally using Jekyll.
- Add info to databases.md on support for less common databases, that were formerly supported as part of Liquibase core.
- Change references to old XSDs on quickstart and index pages.
- Remove XML v2 example from databasechangelog.md, seemed pointless as the only difference was the XSD URL.
- Changed XSD v3.0 to v3.1 in databasechangelog.md.
- Removed list of old XSDs from databasechangelog.md, since it also appears in the xml_format.md page.
- Add Liquibase 3.1 XSD to the list of XSDs on xml_format.md.
- Add a note to the preconditions page, that preconditions at the changelog level apply to all changesets within a changelog, not just the file that contains the precondition.

Lots of formatting fixes:

- Fix use of Markdown syntax within HTML <table> tags - it doesn't work. Mostly affects "required" and "since" emphasis.
- Change invalid double forward-slash syntax into double-asterisks.
- Add header row to a lot of tables
- Fix broken links
- Fix formatting for command-line and properties examples, now contained in syntax highlighting elements.
- Fix some tables that had multiple entries per row, e.g. in preconditions.md.
- Change bullet points in download/index.md to separate sections.
- Fix some bullet points not being rendered due to lack of an empty line before the list starts.