Add tests for functions with return type annotations

[mauro3]

No description provided.

[rafaqz]

+1. Adding a return type to the readme example gives an error in SimpleTraits v.0.6.0, julia 0.6:

@traitfn f{X; IsNice{X}}(x::X)::String = "Very nice!"                                               
ERROR: MethodError: no method matching start(::Void) 
Closest candidates are:
  start(::SimpleVector) at essentials.jl:258
  start(::Base.MethodList) at reflection.jl:560
  start(::ExponentialBackOff) at error.jl:107
  ...                                                
Stacktrace:                                          
 [1] eval_user_input(::Any, ::Base.REPL.REPLBackend) at ./REPL.jl:66                                       
 [2] macro expansion at /home/raf/.julia/v0.6/Revise/src/Revise.jl:775 [inlined]                           
 [3] (::Revise.##17#18{Base.REPL.REPLBackend})() at ./event.jl:73