Test should use local files.

One test depended on files being in their installed locations,
which isn't very useful if `make check` is run before
`make install`. Fixed that to use the local version of the style
file.
Abstract `expire_tiles` output.

Rather than just outputting to a file at destructor time, which
has the potential to go horribly wrong if anything is thrown, this
now needs `output_and_destroy()` to be called before the object
goes out of scope.

The output is now effectively a functor object, so this allows it
to be swapped out for testing purposes.
Fixed use-after-free bug.
Started adding basic expiry tests.
More test infrastructure.
Added expiry tree merging.

This merges the expiry trees of two `expire_tiles` objects,
destroying the second in the process. This can be used to reduce
`expire_tiles` objects created in other processes back onto the
main thread.
Added tests covering more code paths.
Added comments.
Merge pull request #6 from MapQuest/mla_threadpend

Added expiry tree merging