test approximateUniqueCount

This pull requests tests approximateUniqueCount and shows how to pass utf8 -> bytes function there. This feature is not documented and this test tries to solve that.

*Was:*
When I use ```approximateUniqueCount[String]``` in scalding 0.9_SNAPSHOT the import ```import com.twitter.bijection.Injection.utf8``` doesn't work as an implicit because it's a var in bijection 0.5.4

As a workaround I had to create my own
```implicit def utf8(s: String): Array[Byte] = com.twitter.bijection.Injection.utf8(s)```

See the test how it works. If you replace it with the import your will get this compilation error:
```
sbt
> test-only com.twitter.scalding.ReduceOperationsTest

[error] /Users/oiepishkin/projects/github/scalding/scalding-core/src/test/scala/com/twitter/scalding/ReduceOperationsTest.scala:84: No implicit view available from String => Array[Byte].
[error] Error occurred in an application involving default arguments.
[error]         _.approximateUniqueCount[String]('os -> 'os_count)
[error]                                         ^
[error] one error found
```