Force UTF-8 encoding and character replacement

I admit this is probably a hacky fix, and I'm open to suggestions on how
to do this better.

Problem description:

When sending certain characters, Hipchat resets the connection.  This is
a Bad Thing(tm) - Lita does not reconnect, nor does it exit.

Repro steps:

With the lita-whois handler and lita-hipchat adapter installed, do:

```
!whois 194.132.177.163
```

and the adapter will disconnect, and show the following in debug mode:

```
W, [2014-07-18T11:41:45.351138 #58034]  WARN -- : EXCEPTION:
    Errno::ECONNRESET
    Connection reset by peer
    /Users/esigler/.rvm/gems/ruby-2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/connection.rb:45:in `sysread'
    /Users/esigler/.rvm/gems/ruby-2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/connection.rb:45:in `sysread'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/openssl/buffering.rb:61:in `fill_rbuff'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/openssl/buffering.rb:301:in `eof?'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/rexml/source.rb:235:in `empty?'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/rexml/parsers/baseparser.rb:148:in `empty?'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/rexml/parsers/baseparser.rb:196:in `pull_event'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/rexml/parsers/baseparser.rb:184:in `pull'
    /Users/esigler/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/rexml/parsers/sax2parser.rb:92:in `parse'
    /Users/esigler/.rvm/gems/ruby-2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/streamparser.rb:91:in `parse'
    /Users/esigler/.rvm/gems/ruby-2.1.0/gems/xmpp4r-0.5.6/lib/xmpp4r/stream.rb:75:in `block in start'
```

The part of the string in question is:

```
address:        Humleg�rdsgatan 20
```

Forcing the string to be encoded as UTF-8, and replacing invalid or
undefined characters, prevents the Hipchat disconnect.
Merge remote-tracking branch 'upstream/master'
Pull encoding out to a separate function
Merge pull request #12 from esigler/master

Force UTF-8 encoding and character replacement