206 improve def

[jbcaillau]

@def can now also be used without naming the problem defined:

ocp = @def begin
           t ∈ [0, 1], time
           x ∈ R², state
           u ∈ R, control
           x(0) == [ -1, 0 ]
           x(1) == [ 0, 0 ]
           ẋ(t) == [ x₂(t), u(t) ]
           ∫( 0.5u(t)^2 ) → min
       end

• Keeping the previous version for compat. (Also allows log=true mode to print out parsing info.)
• doc updates needed

[jbcaillau]

@ocots

• I have left onepass tests with the old syntax @def o instead of o = @def
• doc to be updated, check doc update: @def new syntax OptimalControl.jl#284
• please review and merge

[ocots]

Youcan make a new release updating the Z.