Use Ruby to decode uuencoded attachments

Ruby can natively decode uuencoded text with String#unpack. [1] This
avoids the uuencode program dependency and avoids writing tempfiles.

The actual implementation is taken from mail [2]. The UnixToUnix module
is not available in our bundled version of mail.

This commit includes a spec to illustrate the failure of uuencode(1) to
decode the particular attachment in incoming-request-bad-uuencoding-2.
email. Parsing with uuencode returns the following error:

    External Command: Error from command "uudecode -o /dev/stdout
	/tmp/foiuu20150530-14811-u6j936":
	uudecode: /tmp/foiuu20150530-14811-u6j936: No `end' line

The file _is_ created and appears to be OK, but the exit code is 1. This
causes AlaveteliExternalCommand to fail and return nil.

See #2508 [3] for the bug report.

[1] http://ruby-doc.org/core-1.8.7/String.html#method-i-unpack
[2] https://github.com/mikel/mail/blob/bc4c9bb9321e9d36a678692f2f562d3146b63f78/lib/mail/encodings/unix_to_unix.rb#L7
[3] https://github.com/mysociety/alaveteli/issues/2508

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