Specify individual long tests to run

The test script currently only allows to run all the long test. This
change will allow a user to specify individual long tests to run.
Update goleveldb to fixed version (dropping data after compaction + recover). syndtr/goleveldb#53
Merge branch 'specify_long_tests' of https://github.com/simonaquino/aptly
Fix system tests.
List snapshots by time

Users now have the choice of listing the snapshot by time as well as
name (default behaviour).
An additional flag has been added '--sort=' which controls the sort
method applied to the list produced by aptly snapshot list.

The possible values are:
--sort=name (default): sorts the snapshot list by name (lexicographic
order)
--sort=time: sorts the snapshot list in chronological order (oldest to
newest)
List snapshots by time: added integration tests

Added a couple of integration tests for the new list snapshot by
creation time feature.
Implementation of all-matches functionality + tests

When performing an *aptly snapshot pull*, users might list dependency
versions that can potentially match multiple packages in the source
snapshot. However, the current implementation of the 'snapshot pull'
command only allows one package to be pulled from a snapshot at a time
for a given dependency.

The newly implemented all-matches flag allows users to pull all the
matching packages from a source snapshot, provided that they satisfy the
version requirements indicated by the dependencies.

The all-matches flag defaults to false and only produces the described
behaviour when it is explicitly set to true.
Solved merge conflicts
Merge branch 'simon_changes_combined' of https://github.com/simonaquino/aptly into simon_changes_combined