Added feature to set path to artifacts in S3, fix for #20, and changes to handling of hidden artifacts

[mmallahan]

I made some changes to this plugin in order to make it work better for my organization:

1. I added a feature to set the path to the artifacts in S3. This supports parameters in TeamCity. It defaults to the equivalent of the existing hardcoded path.
2. I implemented the fix described in Compatibility with TeamCity Windows version  #20, which seems to also work fine on Linux. This means that folder paths won't be mangled when a Windows TeamCity server is used.
3. I changed the code so that only the ".teamcity" folder is excluded from being pushed to S3. This mirrors how only that specific folder is considered "hidden artifacts" by TeamCity. It would probably be a good idea to also add a checkbox to enable/disable pushing this folder, but we don't personally need it.