add #pt2pt to Prawn::Measurements
initial support for pluggable parsers

parser management is done in `Prawn::Text::Formatted`.
i tried to keep everything as simple as possible!
correct interface of to_array method in prawn default parser
different approach: choose the parser when the document is created.
=> new option called `:parser` in `Prawn::Document`
   defaults to the parser shipping with Prawn (`Prawn::Text::Formatted::Parser`).
Merge branch 'master' into pluggable-parsers

Conflicts:
	lib/prawn/table/cell/text.rb
	lib/prawn/text/formatted/parser.rb
Merge branch 'master' into pr/354

Conflicts:
	lib/prawn/font.rb
Change references from parser to text_formatter
Add spec for useing a provided text_formatter
Merge pull request #586 from prawnpdf/pluggable-parsers

Pluggable parsers