Add decoding of IMAP flavor of UTF-7

We have a method for encoding UTF-7 modified for use with IMAP. We
should add additional method for decoding it.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Encode UTF-7 to ASCII with convenience method

Similar to the convenience method that decodes ASCII, we should have a
convenience method that returns an ASCII compatible string.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Parse LIST replies

We can now implement support for LIST replies since the string scanner
support both parenthesized lists and strings, and we can also decode the
IMAP flavor of UTF-7.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>