switch from test-spec to rspec

* test-spec is seemingly abandoned
* the last release was in 2009, before MRI 1.9.2 was released, which
  means it is not encoding-aware
* it's also useful to use a test framework that us familiar to more
  potential contributors (and existing committers with poor memories)
require rdoc on all platforms to ensure the rake task is available
remove iconv from the specs

* iconv is always available on the platforms we officially target
* however it's NOT available on MRI 2.0.0, due for release in February
  2013. We will need to support that soon, so get our house in order
for some reason on jruby 1.7 the encodings here don't always match
these specs must load files as binary to ensure the encoding is marked correctly
MRI and jruby treat encodings differently when reading from a StringIO

* as long as the string that the StringIO wraps is marked as binary,
  then the behaviour will match
update deprecated rspec syntax

* be_close(.., ..) is now be_within(..).of(..)
add an rspec config file
use the ruby-rc4 gem instead of our own internal implementation

* ruby-rc4 is an indirect dependency via pdf-reader anyway
* less code for us to worry about in prawn is a good thing
* ruby-rc4 seems stable and reliable
delegate to the AFM gem for parsing AFM files

* reduces the responsibilities of the Prawn::Font::AFM class. It's about
  adding the font to the ObjectStore and encoding text, not parsing a file
  format.
* allows pdf-reader and prawn to share the same AFM parsing code. More
  eyeballs, less repetition, etc.
* the afm gem is minimal, stable and tested
Merge pull request #427 from prawnpdf/afm

delegate to the AFM gem for parsing AFM files
Merge branch 'master' into rc4

Conflicts:
	Gemfile
	prawn.gemspec
Merge pull request #426 from prawnpdf/rc4

use the ruby-rc4 gem instead of our own internal implementation
Merge branch 'master' into jruby-specs

Conflicts:
	spec/table_spec.rb
Fix : image not properly positioned inside column_box
Merge pull request #425 from prawnpdf/jruby-specs

Green specs on MRI, Jruby and rbx
Merge branch 'master' of git://github.com/sgwanlee/prawn
Maintain correct :Length entry value at all times
Fixed soft_mask spec
Do not explicitly set :Length, streams will take care of that.