Use the Glib slice allocator for fixed-size objects

The slice allocator, available since GLib 2.10, is more
space-efficient than [g_]malloc.  Since BarnOwl is obviously at the
leading edge of space-efficient technology, this seems like a natural
fit.  Use it for every fixed-size object except
owl_viewwin_search_data (which would need an extra destroy_cbdata
function to g_slice_free it).

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Use SSL by default

As per https://dev.twitter.com/discussions/24239 and
https://twitter.com/perl_api/status/423210390684565504, the Twitter API now
requires SSL connections, so go use them.
squelch the legacy_list_api warnings
Kill the client-side tweet length check.

Just let the API do the check. Since we don't shorten URLs, our check
is way wrong anyways.
Implement :twitter-favorite
Merge pull request #160 from nelhage/twitter

Some misc twitter fixes and improvements