RFC : Alternative implementation of timedwait

This is an alternative to @timedwait as implemented https://github.com/JuliaLang/julia/pull/3764 

Differences: 

- ```timedwait``` is a function

- polling interval is a kw arg with default of 0.1 seconds

- code moved to ```multi.jl``` from ```util.jl```

Return values are still symbols instead of integer values. Symbols are self explanatory and do not need to be exported. 

Do let  me know if this is preferred over the macro implementation. Wanted this issue closed before I document and use the same elsewhere.