Mitigated a performance regression due to the introduction of the location information

Performance of kramdown on Ruby 2.0 and 2.1 got 4x resp. 3x
worse due to storing of the location information. Prior
Rubies are not affected that much.

With this fix, performance is only 2x worse on Ruby 2.0 and
0.5x on Ruby 2.1
Added prawn as development dependency
Updated performance graphs
Updated release notes, version and homepage