Add support for UTF-7 encoding

UTF-7 is rarely used but still it can be found in some emails.

For example this email is encoded using UTF-7:
http://lists.jboss.org/pipermail/rules-dev/2012-October/004213.html

The source message header contains:
Content-Type: text/plain; charset="UTF-7"

The problem is that by default Java does not support UTF-7 (yielding UnsupportedEncodingException). To solve this we need to bring in third-party library for UTF-7 support, for example:

<dependency>
<groupId>com.beetstra.jutf7</groupId>
<artifactId>jutf7</artifactId>
<version>1.0.0</version>
</dependency>