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