relaxing aws credentials check to allow dynamic establishing of
credentials through roles
Removed storing of credentials and replaced all SDK calls to use
unparameterized constructors to follow the AWS order to establish the
credentials. If credentials are set explicelty, such will be exported
into the AWS specified system properties and can then be used as such by
the AWS SDK. This allows using Role based permissions and letting the
SDK automatically reestablish the credentials once rotated by the
Instance Metadata Service. 
changed properties check from NotEmpty to NotBlank, to cater for
accidental whitespaces
Removed previously deprecated methods; added javadocs to
AWSClient constructor about credentials acquisition; added comment to
clients.properties to mention Instance Roles configuration.
Merge pull request #28 from DrGranit/master

Adding Role policy handling in addition to explicitly set user credentials