Added dynamic region retrieval, API based tag filtering and list_all_tagged_nodes.ps1.

[sopel]

To get into AWS usage via PowerShell some more I've adjusted the following aspects:

• added dynamic region retrieval
• replaced client side tag filtering with API based tag filtering
  • the AWS API provides filters for most describe*() methods, which can be used to reduce the retrieved resources API side already, and offers versatile filter combinations as well - see Listing and Filtering Your Resources for details
  • the more flexible approach would accordingly be to allow specification of the entire filter as an argument, but for he use case at hand the simple tag name variation seems more appropriate (an optional value rather than the hardcoded '*' would add some flexibility of course)
• added list_all_tagged_nodes.ps1 to test/experiment with the filtering
• fixed *.InstanceId.Count printing

[sopel]

Just realized that the target pull request #103 has been closed and merged into master already. I'm leaving this one open against the branch still for review/merge by #103 owner @mrdavidlaing, it can be merged to master from there in case.