[command] Implement SPARQL document parser and add support for #INCLUDEs

First version of the new 'Execute SPARQL Query' command that now offers
support for section metadata and ... ratatatam ... #INCLUDEs. Yay!

By using Includes, one can reuse named sections all over the place.
This way, it is possible to write typically used snippets once, and
then just "call" them from anywhere else when needed. This is
especially helpful for SPARQL beginners or when iteratively developing
complex queries where one wants to see the effect of a previous query
immediately.

When executed, every included snippet is listed separately in the web
preview window of TextMate. That way it is easy to track down issues
with intermediate states (and not only after all queries just have
finished).
[snippet] Add some SPARQL snippets
[cosmetics] Add SCSS document for web preview of "Execute SPARQL Query"
[command] Remove needless require calls from 'Execute SPARQL Query'
[command] Assert endpoint URL follows the http/https scheme (Execute SPARQL Query)
[command] Make Turtle::SPARQL.get_endpoint return not the Marker object itself but its value
[command][fix] Make Turtle::SPARQL::Section#first_endpoint return not the Marker object itself but its value

and revert Turtle::SPARQL.get_endpoint to its previous version
[command] Rescue from Errno::EHOSTUNREACH and present user a more descriptive message
[cosmetics] Remove puts line
[snippets] Add snippets for SPARQL markers

+ Snippets for #INCLUDE, #QUERY and #UPDATE
[command] Toggle all query logs at once by holding shift pressed while clicking on the link
[command] Improve legibility of HTML tables by tweaking its CSS
[command][fix] Replace wrongly used String#chomp! with non-banged variant

Made the 'Execute SPARQL Query' command fail on lines that contained
nothing that could be chomped
[command] Display number of executed queries in title of preview window
[README] Updated README to reflect recent changes