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.
Merge smira/master into pull_multiple_packages

Resolved conflicts arisen following smira's new commits into master.
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.
Merge branch 'sort_snapshots_time' of https://github.com/simonaquino/aptly into simonaquino-sort_snapshots_time
Allow running system tests by mask.

E.g. system/run.py 'ListSnapshot*'.
Update man page. #73
Add system test on wrong --sort parameter. #73
Add --capture to auto-create 'gold' results when fail.
Slight refactoring, make wrong param real error. #73
Merge branch 'simonaquino-sort_snapshots_time'
Fix system tests (whitespace).
Merge branch 'pull_multiple_packages' of https://github.com/simonaquino/aptly into simonaquino-pull_multiple_packages
Revert changes related to NextVersion. #70

It would be done in a bit different way: by introducing powerful search queries.
Simplification: we are already able to search for all packages. #70
Dependencies should be matched for each package one by one. #70
Remove 'allMatches' on version equal. #70
Refactoring: simplification. #70
Update man page. #70
System tests for aptly snapshot pull -all-matches. #70