update docs for 0.3.1
clarify callback support
Added, documented push and append for GenericAffExpr
try to fix travis
Merge pull request #95 from JuliaOpt/affappend2

Added, documented push and append for GenericAffExpr
Fix redundant word in affexpr docs
detect ranges with steps. fixes #93
Merge branch 'master' of github.com:JuliaOpt/JuMP.jl
add tests for #93
rebase?