ci: Add .travis.yml for continuous integration
Merge pull request #28 from gpoo/master

ci: Add .travis.yml for continuous integration
ci: Add current Travis status on README file
utils: Accept gzip enconding downloading files. Fix #11

In addition, files with compressed extensions, but not compressed,
are now correctly analyzed.
Merge pull request #27 from sduenas/gzip-mboxes
Refactor remote url retrieval files

Make available only one method to retrieve a web resource,
instead of one for parsing the html index and another one
for the mbox.

In addition, added magic numbers to detect the file type
by content, which is better than guessing by the subtype
given by the web server (not always available).

Signed-off-by: Santiago Dueñas <sduenas@bitergia.com>
Attempt to parse multiple messages in mbox and fix #10