Add Shared Credential Provider

Adds a credential provider to the default stack which looks for
credentials in the `~/.aws/credentials` file, and will use those after
checking the AWS#config parameters and ENV but before checking EC2.

Includes an INI parser to parse the included configuration file.
Refactored the new SharedCredentialFileProvider.

Changes include:

* Constructor now accepts an options hash with :path and :profile_name
* Extracted internal methods of SharedCredentialFileProvider for readability
* Removed test requirement for access to Dir.home
* Removed puts debugging statement from the default provider
Added support to AWS::SQS::Client for message attributes.
Fixed an issue with Ruby 1.8.7

Enumerable#with_object not supported in 1.8.7, replaced
with #inject.
Bump version to v1.40.0