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
Merge branch 'master' into jruby-specs

Conflicts:
	spec/table_spec.rb
Merge pull request #425 from prawnpdf/jruby-specs

Green specs on MRI, Jruby and rbx