Green specs on MRI, Jruby and rbx

Hola,

This branch updates prawn's test suite to be green on:

* MRI 1.8.7, 1.9.2, 1.9.3, 2.0.0preview1
* Jruby 1.8 and 1.9 modes
* rbx 1.8 mode

The build is still red on rbx 1.9 mode due to encoding issues in their VM.

There are two small (and very ugly) changes in lib. The remaining changes are all in the Rakefile and spec/*.

The most controversial change is a switch from test-spec to rspec. The changes are fairly cosmetic, but we should discuss if it's a direction everyone is happy with. test-spec was released in 2009 and appears to be ignorant of encodings. It works OK on MRI, but it was causing a lot of the encoding related errors on jruby. See this bug i filed with jruby [1].

I feel like it's better to be using a test framework that is actively maintained and will work with all rubies as they evolve, and switching to rspec was less invasive than test-unit.

James

[1] http://jira.codehaus.org/browse/JRUBY-7021