Begin to add `only` option
Fixes the #silenced? method when "only" patterns are present

As soon as a path is matched against an "only" regexp, it's
not silenced, so it's silenced if it doesn't match any "only"
regexps.
If fixes the previous logic that was requiring a path to match
all "only" patterns in order to not be silenced, meaning that
it was silenced as soon as it didn't match an "only" regexp
(even if it matched another "only" regexp).
Merge pull request #160 from guard/only-fix

Fixes the #silenced? method when "only" patterns are present
Check for silenced path only against files

One of the reason for this change is that in the end the changes we
report to the user (i.e. in the changes callback) are files changes,
not directories changes. And also because it's simpler to implement
this way.

Note: A `only_dirs` option would have a better performance score (since
it could skip entire directories instead of scanning each file to check
if they match the `only` regex or not).
Fix specs
Add debug print for Travis
Fix change type in bsd, linux and windows adapter

‘file’ instead of ‘File’
Merge pull request #151 from guard/only

New `only` option