Closes #338

In case of non-existing user return cb(false) as expected.
Merge pull request #339 from alessioalex/patch-1

Closes #338
Implement line length limits
Document max_line_length and max_data_line_length
Make line length limits configurable
Note transaction.notes.data_line_length_exceeded
Merge pull request #340 from baudehlo/line_length_limits

Implement line length limits
Merge branch 'master' of git://github.com/baudehlo/Haraka