the iconv library is deprecated in ruby 1.9.3

it prints a warning every time it comes across this:

```ruby
require 'iconv'
```

iconv is only being used in one place in the calais gem, in Client#enlighten, to convert a string into valid utf-8, ignoring errors. i've migrated this to use ruby 1.9's String#encode, with a fallback to the original code in case the user is running ruby 1.8.

this has silenced the warning and is giving me the same behavior i had before. let me know if you'd like any other changes made to this before inclusion.

regards