Move default timeout from MetadataService to EC2 cred provider

The AWS.EC2MetadataCredentials now has a 1000ms timeout.
The AWS.MetadataService has a default 0ms timeout.

Also updated documentation to reflect these changes and provide more
information on how to change timeout values.
Merge pull request #368 from aws/fix/metadata-service-no-timeout

Move default timeout from MetadataService to EC2 cred provider