Make Timezone and Country responsible for resetting caches in tests.

This also has the side effect of stopping RDoc from determining that
ThreadSafe is a child module of TZInfo::Timezone and TZInfo::Country.
Move RDoc options into gemspec.
Don't require 'rdoc'.

Requiring 'rdoc' causes an error running the rdoc task on Ruby 1.9.3
when the rdoc 4.0.1 gem is installed.