Fix the webfeed import for the new Go appengine dev runtime.

The new runtime uses the directory containing app.yaml as a root of GOPATH. Specifying the github.com/velour/feedme/webfeed path was causing issues, because the runtime found two conflicting versions of webfeed.
Update TestFixHtml.
Use some simple heuristics to make summaries look good as text.

Strip HTML and replace images with their alt text. If all else fails, just replace the description with a link.
Merge pull request #172 from eaburns/dev/eaburns

Attempt to ensure that summaries show well as text
Use the non-good-as-text description if there is no content.

Some feeds have no content; they use their description instead. But, they don't want the description text-ified. So don't.
Merge pull request #174 from eaburns/foobarbaz

Use the non-good-as-text description if there is no content.