Clean up CensorRule

- Avoid self when not required
  https://github.com/bbatsov/ruby-style-guide#no-self-unless-required
- Pad hashes
- Line length
- Omit paentheses for method calls with no arguments
  https://github.com/bbatsov/ruby-style-guide#no-args-no-parens
- Avoid return where not required
  https://github.com/bbatsov/ruby-style-guide#no-explicit-return
Extract duplication in CensorRule
Privatise uncalled CensorRule methods

Make public api as slim as possible
Avoid use of parameter with attribute name

The `text` parameter in `apply_to_text!(text)` can easily be confused
with the `text` attribute of a CensorRule. Renamed to avoid confusion
and renamed the parameter in `apply_to_binary!` for consistency.
Merge branch 'censor_rule_cleanup' into rails-3-develop