AS::MB::Chars#slice! doesn't operate in-place; guide docs are out of date

The patch I will attach fixes a few remaining issues with AS::Multibyte::Chars:

• #slice! was not operating in place
• On 1.9, #strip!, #lstrip! and #rstrip! were going through the proxy class, but the corresponding bang-less methods were not. The patch avoids defining these methods on 1.9 since they are already multibyte-aware.
• The Rails initialization guide needed to be updated to reflect recent changes

I also took the opportunity to improve the test setup; I used class_eval to generate single-assertion tests for the bang methods, rather than doing a bunch of assertions in a loop. This makes test failures a little easier to track down.

Edited by Rohit Arondekar for formating.