Fixed a bug related to irregular filenames.

The mail library encodes filenames with consecutive whitespace
such as 'a  b.txt' as 'a b.txt', which causes a null pointer
exception in Pony.add_attachments. This patch changes the
filenames used internally to match those in the mail library.
Nested MIME multipart/alternative block for mixed body type with attachments.

The mail library fails to properly distinguish file attachments from
the message body when there are both text and html versions present,
instead listing the attachments as another version of the body in
another alternative format. Where all three are present it is
necessary to explicitly define a separate MIME boundary which
distinguishes body versions from attachments.
Fixed encoding of html part.

HTML part was listed as quoted-printable but was not being encoded
as such, leading to a bug where any = symbol in the text would
cause the body to be truncated.
Removing content_transfer_encoding.

Added this in an early attempt to fix the problems we've been having.
Tidy up.
Merge pull request #13 from dslh/master

Nested multipart boundaries for emails with attachments and body and html body