Added support for IIS pickup directory.
listed `pickup` in the readme.
added `transport.close()` tests for pickup type.
checked if the specified directory exists.
disabled pickup tests - need some mocking.
Merge pull request #228 from philipproplesch/master

Store mails in a directory for later processing
Changed pickup argument name to 'directory'
Bumped dependency versions for mailcomposer and readable-stream
Return messageId with responseObject
Allow defining custom transport methods
Pumped version to v0.6.0
Merge pull request #14 from andris9/master

上游合并