Update gemspec and Rakefile

Streamlined the installation process a bit.

The gemspec is updated to latest standards - especially not specifying the dependency to `sparql-client` breaks this. The `require_paths` are needed so that the gem can work within other applications that require it as dependency.

bundler's `gem_tasks` have been added to build, install or release the gem.

`rake server:install` installs fuseki, `rake server:start` starts it.

Installation of the gem is not needed to get it running as the gemspec requires the lib folder correctly now - it can therefore used from within the repo. This further simplifies the README - you sent the users off to `/usr/local`, but that won't be true for everyone - I am an rvm user, so I won't find installed gems there.