access linear constraint from ConstraintRef

[mlubin]

Instead of a new export as suggested in #462 (comment), just add a new constructor for LinearConstraint. If we want a more generic version, we can do

getConstraint{T}(ref::ConstraintRef{T}) = T(ref)

on top of this.

Can be implemented for other constraint types (as needed).

@FabrizioLacalandra, you may be interested in this

[mlubin]

Added some docs, ready to merge.

[joehuchette]

👍

[mlubin]

Homebrew's not working on julia nightly? @staticfloat @tkelman

WARNING: src::AbstractCmd |> dest::AbstractCmd is deprecated, use pipeline(src,dest) instead.
 [inlined code] from error.jl:26
 in depwarn at deprecated.jl:73
 in |> at deprecated.jl:50
 in install_brew at /Users/travis/.julia/v0.5/Homebrew/src/Homebrew.jl:80
 in init at /Users/travis/.julia/v0.5/Homebrew/src/Homebrew.jl:26
 in include at /Users/travis/julia/lib/julia/sys.dylib
 in include_from_node1 at /Users/travis/julia/lib/julia/sys.dylib
 in require at /Users/travis/julia/lib/julia/sys.dylib
 in include at /Users/travis/julia/lib/julia/sys.dylib
 in include_from_node1 at /Users/travis/julia/lib/julia/sys.dylib
 [inlined code] from sysimg.jl:14
 in evalfile at loading.jl:320
 in evalfile at loading.jl:320
 in anonymous at none:8
 in cd at file.jl:22
 in anonymous at none:7
 in open at iostream.jl:114
 in process_options at /Users/travis/julia/lib/julia/sys.dylib
 in _start at /Users/travis/julia/lib/julia/sys.dylib
while loading /Users/travis/.julia/v0.5/Homebrew/src/Homebrew.jl, in expression starting on line 353
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  258k  100  258k    0     0   303k      0 --:--:-- --:--:-- --:--:--  303k
WARNING: Could not tap staticfloat/juliadeps!
==============================[ ERROR: Homebrew ]===============================
LoadError: LoadError: pipe is already initialized
while loading /Users/travis/.julia/v0.5/Homebrew/src/Homebrew.jl, in expression starting on line 353
while loading /Users/travis/.julia/v0.5/Homebrew/deps/build.jl, in expression starting on line 1

[tkelman]

possibly due to JuliaLang/julia#13506 ?