Upgrade AWS::EC2::Client to latest API version.

Upgrades AWS::EC2::Client to 2013-08-15 API version. Includes modifications to
Amazon EC2 Reserved Instances API.
Bump version to 1.18.0
Update EC2 Version in README
Add parameter validation to put_bucket_cors

Adds validation that two required parameters in AWS::S3::Client#put_bucket_cors
are present and match the type requirements.

Resolves Issue #357
Add yard-sitemap to Gemfile, update rdiscount gem location
Use yard-sitemap plugin when generating documentation
Use 1.8 syntax in Gemfile
Add SITEMAP_BASEURL to docs task
Give Bundler a hint about which Nokogiri version to use.

Bundler seems to have problems resolving version conflicts with
less-than constraints (as per bundler/bundler#2593), so provide a
greater-or-equal-to constraint as well to give Bundler a hint.

Closes #361
fix documentation: replace 'alarm' with 'change'
Merge pull request #364 from mkrogemann/master

fix documentation: replace 'alarm' with 'change'
Add documentation for setting Content-Disposition in copy_from

#66 added support for setting the Content-Disposition header in `copy_from`, but the documentation didn't reflect that this new option was available.
Merge pull request #366 from dmarkow/patch-1

Add documentation for setting Content-Disposition in copy_from
API Updates for Auto Scaling
Bump version to 1.19.0
Update nokogiri dependency to 1.6.0
Merge branch 'master' of github.com:leeh/aws-sdk-ruby