Fixed bug in #28. Modified MailCommand class to avoid populating the sender field if it's empty per RFC5321 specifications.
Added a unit test class SMTPCommandTest to test socket level commands to GreenMail SMTP server.
Merge pull request #31 from buildscientist/issue-28

Fixed bug in #28. Modified MailCommand class to avoid populating the sen...
Starting fixing NOT search term: cleanup
Fixes #35 : Supports NOT search term
Fixes #35 : Do not throw exception for unsupported search term but warn instead
Sonar cleanup
Added travis-ci build status
Removed duplicate ci status