Archive only compressed files. Fix #2

The tool stored mbox files twice: compressed and uncompressed.
With this commit, downloaded contents are only stored compressed
(even when they are uncompressed on the origin).

MBoxArchive instance provides a new property container() which
returns a container to the compressed archive. This container
is a file descriptor that will decompress the content on the fly.
This means that MailArchiveAnalyzer does not recive a filepath to the
mbox anymore. It gets a a MBoxArchive object to parse the content from
it.

Unit tests were also updated.
Fix error retrieving local mboxes
Store real path instead of the relative path for local mboxes
Merge pull request #34 from sduenas/compressed-mboxes

Archive only compressed files. Fix #2