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.
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
Merge branch 'simonaquino-pull_multiple_packages'