Add basic support for linking to local files.
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`).
Failing test to proof the same font objects aren't equals
Implemented Font#hash and Font#eql?
Removed duplicate methods in Font
Pure whitespace removal
Revert "Pure whitespace removal"

This reverts commit 258495979060bd64b61a6cb942c01fa2f1b6abf6.
Much better Font#eql?, slightly better Font#hash
Work around Font#size being private
Extracted two paths of Prawn::Document#width_of into private methods
Removed broken thread-local cache in Prawn::Table::Cell::Text#styled_width_of_single_character
Naive font metric cache implementation
Failing test to proof the same font objects aren't equals
Implemented Font#hash and Font#eql?
Removed duplicate methods in Font
Pure whitespace removal
Revert "Pure whitespace removal"

This reverts commit 258495979060bd64b61a6cb942c01fa2f1b6abf6.