minor import/using improvements

This patch adds the following features:

1) Import several functions from the same module w/o retyping the module name:

```jlcon
julia> import min, max from Base

julia> min() = 0
# method added to generic function min

julia> min()
0

julia> import from from Base
Warning: could not import Base.from into Main
```

2) Use only specific functions in a module:

```jlcon
julia> using Elliptic.Jacobi.sn

julia> sn
# methods for generic function sn
sn(u::Float64,m::Float64) at /home/nolta/.julia/Elliptic/src/jacobi.jl:50
sn(u::Real,m::Real) at /home/nolta/.julia/Elliptic/src/jacobi.jl:81

julia> sn() = 0
ERROR: error in method definition: function Jacobi.sn must be explicitly imported to be extended
```