Renamed the `toJSON` method to `build` for the JSON.Builder.

Renamed method to avoid conflicts with other libraries.

Fixes #186
Update Redshift API
Update IAM API to support SAML management operations
Add support for AWS CloudTrail (AWS.CloudTrail)
Add integration tests for AWS.CloudTrail
Add AWS.STS.assumeRoleWithSAML()
Allow STS.assumeRoleWithSAML to work without access key
Add AWS.SAMLCredentials credentials object

Allows users to configure credentials for a SAML-enabled
identity provider. Usage requires a RoleArn, PrincipalArn, and
base64-encoded SAMLAssertion token from the identtiy provider:

```javascript
AWS.config.credentials = new AWS.SAMLCredentials({
  RoleArn: 'arn:aws:iam::123456789:role/Role',
  PrincipalArn: 'arn:aws:iam::123456789:role/Principal',
  SAMLAssertion: 'TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5v=='
});
```
Firefox always appends charset=UTF-8 in Content-Type header.

Fixes #190
Support Request.abort() in the browser

Fixes #192
Update AWS.RDS to 2013-09-09
Update README with new API versions
Merge branch 'master' into v1.x-stable
Bump version to 1.13.0