allow get_filter_value to return state of ami and test

  * updated Ami.get_filter_value to return the state value of the AMI
  * updated test_ec2.test_amis.test_ami_filters to test for AMIs in the state of 'available'
Test Fixes and `tag:` filter.

  * fixed test_ec2.test_amis.test_ami_filters to test for AMIs in the state of 'available'
  * enhanced AMI's to be able to take `tag:` filter.
  * added tag: tests for AMIs
Merge pull request #219 from joekiller/ami_filter_state

allow get_filter_value to return state of ami and test