Change required ruby version 1.9.3 -> 1.8.7

Semi-reverts part of commit d9b59ff.
Updated rake:rdoc task.

Changed "README" -> "README.rdoc".
Add rdoc and rake to gemspec dev dependencies
Add .travis.yml for testing on MRI 1.8.7
Add Gemfile for travis to install dependencies
Merge pull request #12 from sshao/1.1.1/downgrade_ruby_version_dependency

downgrade ruby version dependency
Merge branch 'master' into remove-xml-ruby-link

Conflicts:
	.travis.yml

Conflict was that master .travis.yml only contained 1.8.7 in the
rvm list, and remove-xml-ruby-link .travis.yml added more
rubies in.
require date in gemspec

should not assume that date has already been loaded