core: Fix backwards pagination in GDataQuery

If using pre-specific next/previous URIs, the previous URI wasn’t being
detected correctly due to a copy–paste error.
youtube: Add a CHANNEL_REQUIRED error code

This is returned for operations which require the currently authenticated
Google Account to have an associated YouTube channel, but it doesn’t.

This adds the following API:
 • GDATA_YOUTUBE_SERVICE_ERROR_CHANNEL_REQUIRED
tests: Update upload limit on PicasaWeb tests

Looks like quotas have been increased from 1GiB to 15GiB!
core: Remove an unnecessary message unpausing

See: https://bugzilla.gnome.org/show_bug.cgi?id=703461#c4
documents: Fix default value of GDataDocumentsUploadQuery:convert

Also add a test for it, because tests are good.
build: Update git.mk from upstream
build: Use AM_C[PP]FLAGS instead of INCLUDES

Let’s live in the future.
build: Use AC_CONFIG_AUX_DIR

This tidies away various autoconf/automake files so they don’t pollute
the root directory. It also ties in with recent git.mk improvements so
these files are automatically included in .gitignore.
build: Update MAINTAINERCLEANFILES
tests: Fix a memory leak in the PicasaWeb unit tests
tests: Work around distributed system bugs in the Google Contacts servers

Looks like the servers have changed to use weak synchronisation of new
contacts, so querying for contacts immediately after adding them now
often won’t return the new ones, or will return them with outdated IDs.
‘Fix’ this by adding a mandatory sleep after creating the contacts, and
before querying them for the first time.
core: Allow HTTPS port to be overriden using the LIBGDATA_HTTPS_PORT envvar

This will be useful for unit testing, allowing network traffic to be
redirected to a local unit testing server.
core: Refactor assertions slightly

This tidies up the code a bit. No behavioural changes.
tests: Add more tests for GDataQuery

This pushes its test coverage up to 92.3% of lines, and 94.6% of functions.
tests: Fix kinds of uploaded documents used in the Documents test suite

I’m sure this worked before, so it looks like Google has tightened up
input validation a bit. Not a bad thing.
documents: Minor code tidy-up
tests: Fix document title comparison

Google changed the semantics of document titles again.
core: Fix race condition in upload thread instantiation

The upload thread takes a reference on the GDataUploadStream, but the
stream could be finalised while the thread is still being created;
hence once the thread main function starts and calls g_object_ref(),
the GDataUploadStream has already been destroyed.

Move the g_object_ref() call to before the thread instantiation to
fix this.
tests: Various distributed system workarounds for the Contacts tests

This also, sadly, includes disabling a few tests which I can’t figure out
how to fix sanely at the moment.
tests: Disable some invalid comparisons in the PicasaWeb tests

Turns out that Google servers can arbitrarily change a few more values
than I thought, which the tests didn’t expect to change.