Add nio File operations

The functionality mirrors the functionality in the io package with the exception that all operations are performed asynchronously using the java nio2  framework.
fixes cause exception propagation
Document transducers that use mutable objects
Implement distinctConsecutiveBy in terms of filterBy2
Test examples in ScalaDoc with sbt-doctest
Merge pull request #229 from scalaz/fix-cause-stack

fixes cause exception propagation
Merge pull request #242 from fthomas/filterBy2

Implement distinctConsecutiveBy in terms of filterBy2
Merge pull request #248 from fthomas/sbt-doctest

Test examples in ScalaDocs with sbt-doctest
Merge pull request #241 from fthomas/doc-mutable

Document transducers that use mutable objects
rename `niofiles` object to `file` per request by @pchiusano
Merge pull request #212 from bryce-anderson/topic/niofiles

Add nio File operations