email-gravatar: fix html syntax issues

an attribute value specification must be an attribute value literal
unless SHORTTAG YES is specified
t0111: Add basic tests for Lua filters

* Validate the email filter by manipulating stdin. Additional checks for
  all the arguments can be added in a later patch.

* Add the exec prefix to all informational messages.

* Rename the filter repository to filter-exec. The Git repository itself
  is not renamed since it can be shared amongst all filter types.

* In the filter checks, check whether all arguments are passed properly
  instead of validating the buffer/stdin only.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>