Model tools: Add various utility functions (#179)

* Add tools for basic utility functions

* Add tools for basic utility functions

* Add Constant Frisch labor utility

* Add elliptical labor utility

* Fix bugs and add tests

* Add new tests

* Change how to evaluate values less than 1e-10

Add minimum version for Optim and NLopt (#170)

* minimum version of Optim is 0.7.3

* minimum version of NLopt is 0.3.4

Latex rendering in docs + other improvements (#168)

* add latex rendering to compute_fp.jl

* update link in discrete_rv.jl

* add latex to estspec.jl

* code highlight in discrete.rv.jl

* add latex renedring to kalman.jl

* fix link in lae.jl

* add latex rendering to lqcontrol.jl

* add latex to lqnash.jl

* add latex to lss.jl

* add latex tp matrix_eqn.jl

* small change to lss.jl

* fix highlighting in lqnash.jl

* Add latex to quadsums.jl

* add latex to robustlq.jl

* small change to kalman.jl

* consistent highlighting in lqcontrol.jl

* small changes to ddp.jl

* small changes to ddp.jl

* add latex to markov_approx.jl

* add latex to mc_tools.jl

* Add highlighting to random_mc.jl

* small fix to lqcontrol.jl

* fix formatting in interp.jl

* Fixes to mc_tools.jl

* fixes to ddp.jl

* Fix formatting in robustlq.jl

* random fixes

* small fix

* add space

* Update discrete estimation doc

ENH: use rand instead of draw for drv sampling (#164)

* ENH: use rand instead of draw for drv sampling (closes #162)

* Add `rand!(::AbstractArray{<:Integer}, ::DiscreteRV)` method

TYPO: actually fix typo...

Merge pull request #158 from QuantEcon/fix_mvn_sampler

BUG: Fix MVNSampler

Merge pull request #154 from QuantEcon/sl/0.6

Sl/0.6

QuantEcon v0.9.0 [d028c89]

QuantEcon v0.8.0 [29de48b]

QuantEcon v0.7.0 [5b7ba94]