Fix recursion when listing directories

Sftp::scanDirectory() always recursively scans a whole directory tree even if no recursion is wanted. This patch uses scandir instead of the recursive method-call to avoid this.
Merge branch 'master' from Herzult/php-ssh into sft-recursion-fix
First try on functional testing.
There seems to be invalid apt-cache entries.
Need sudo to change own password.
Added some tests for Exec. Somehow the STDERR Stream does not seem to work. Skipped that test so far.
Fix .travis.yml
Fix .travis.yml
Merge pull request #20 from woiwa/sft-recursion-fix

Fix recursion when listing directories.
Added functional tests for Sftp.
Needed to silence a few ssh2* function, because they were making undocumented warnings.
Rebased to current master
Fixed wrong default parameter for Exec::run, that disabled the reading of STDERR output.
Unskipped the stderr test.
Not using 'php' binary to create stderr output, because itseems to be not available on travis.
PHP 5.6 is no allowed failure anymore.
Merge pull request #27 from Herzult/dev-functionaltests

Functional tests added using travis-ci platform.