Fix #17 -- update README.md

* Mention AudioIO
* Update the documentation to include wavplay
Address lint warning about shadowing base functions

/Users/daniel.casimiro/.julia/v0.4/WAV/src/WAV.jl:424 [compress_sample_mulaw] WARN   "sign" as a local variable might cause confusion with a synonymous export from Base
/Users/daniel.casimiro/.julia/v0.4/WAV/src/WAV.jl:432 [compress_sample_mulaw] WARN   "exponent" as a local variable might cause confusion with a synonymous export from Base
/Users/daniel.casimiro/.julia/v0.4/WAV/src/WAV.jl:460 [compress_sample_alaw] WARN   "sign" as a local variable might cause confusion with a synonymous export from Base
/Users/daniel.casimiro/.julia/v0.4/WAV/src/WAV.jl:469 [compress_sample_alaw] WARN   "exponent" as a local variable might cause confusion with a synonymous export from Base