Make cgo iconv package optional

To make things more portable, make use of "gopkg.in/iconv.v1" import optional, since not every system has the GNU iconv library on their system.

To use the cgo iconv package, the end-user of the API would need to import with an underscore:

_ "github.com/flashmob/go-guerrilla/mail/iconv"

Otherwise, default to using the stock charset converter in golang if above package not imported.

Todo:

Create the new mail/iconv package and setup an init hook to use iconv whenever it's imported
update envelope.go to use the default charset converter in golang
update tests

This should solve issues

flashmob / go-guerrilla