bug 1066058; add Support Reason API crashstorage type

*Preamble: This PR is primarily submitted for discussion purposes (I expect more work to be necessary).*

This adds the `SupportReasonAPIStorage` class which is a sub-class of `BotoS3CrashStorage`; functionally speaking, the former is basically a *highly lossy* implementation of the latter.

cc @twobraids 
This seems overly verbose. To have a staticmethod with a private method name all to just return a string. Can't this be a configurable thing instead? Using configman. Something like 
```python
class SupportReasonAPIStorage(BotoS3CrashStorage):
    required_config = BotoS3CrashStorage.get_required_config()
    required_config.add_option(
        'bucket_name', 
        default='mozilla-support-reason',
        doc='this must be unique for the whole S3 cluster'
    )
```