Check that keys exist before recursing in build_table

It's not safe to assume that all dictionaries in a list have
the same keys.  For example, with describe-snapshots,
if a snapshot has no tags, then the Tags key will not be
in the dictionary.

Fixes issue #79.
Merge branch 'fix-issue79' into develop

* fix-issue79:
  Check that keys exist before recursing in build_table