Added instance_type support to instances
Added subnet_id support to instances
Added key_name support to instances
Allow passing security groups by ID when creating instances
Consistent _get_multi_param() function in responses

This abstracts _get_multi_param() into BaseResponse and makes it
always ensure that the string it has been given ends with a '.'.  It
had been implemented in three different places, and in use it rarely
postpended a trailing period, which could make it match parameters it
shouldn't have.
Merge pull request #126 from stpierre/support-instance-properties

Support instance properties