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
Encode LiteralString objects

Fixes #320
Encrypt time
Use bin_string helper to simplify specs a bit
Fixes #509, renders directly to file rather than to memory when using render_file.
Whitespace cleanup
Implemented full PDF dash capabilities

The length parameter of the #dash method can now be an array that
specifies a dash/gap pattern.
Updated dash examples in manual to show new full PDF dash capabilities
Merge branch 'master' into full-pdf-dash-capability
Set initial x coordinate for header cells.
Merge pull request #563 from gettalong/full-pdf-dash-capability

Full PDF dash capability
Revert "Refactor so images via Pathname are read in binmode"

This reverts commit 6a05a4ea6e22b43bb0773f7e7b1f7936a6a431b6.
Merge branch 'master' of github.com:prawnpdf/prawn
Merge branch 'master' of https://github.com/basvk/prawn into leak
Merge branch 'master' into pr/354

Conflicts:
	lib/prawn/font.rb
Change references from parser to text_formatter