Added location information to Kramdown::Element.

* Uses StringScannerKramdown (which has current_line_number method) instead of StringScanner.
* Records :location option on all Kramdown::Elements where this makes sense and where it is easily implemented.
* Initializes nested StringScanners with their parent Element's :location as start_line_number.
* Adds tests for :location information.
* Adds tests for StringScannerKramdown.

See pull request for more details.
Some changes to the location feature

* StringScannerKramdown → Kramdown::Utils::StringScanner
* The class now takes the start line number as optional second
  argument.
Disable warnings on test runs because of Prawn
Fixed PNG generation with gnuplot
Fixed problem with uninitialized start line number
Updated release notes, version and homepage
Added direct PDF output to main landing page image
Added missing arrow
Added travis configuration file
Some travis fixes
Some more travis changes
Add rake dev:gemspec task to allow users of bundler to use a local (or, if the gemspec is commited) a specific remote version of kramdown..