Added WAVArray and writemime methods

As previously discussed in JuliaLang/IJulia.jl#247, I added a `writemime` method for playing Julia arrays as WAV files in an IJulia notebook. I created a type (now called `WAVArray`, in lack of a better name) that simply wraps an array and a corresponding sampling rate. The method `writemime(io, MIME"text/html", x::WAVArray)` will then use `wavwrite` to render the vector as a WAV file and embed it in HTML5.

Since this was implemented as a `writemime` for a specific type, any function or code that returns a `WAVArray` in the last line of a notebook cell will render the audio player. A user can optionally call `display(x::WAVArray)` to  render the audio player as well.