Final 0.6 compatible release

This is the final release that will support Julia 0.6.

New features

• Added HP and Hamilton filters. (thanks @Shunsuke-Hori, ref #205)
• Added kalman smoother and log-likelihood for Kalman (thanks @Shunsuke-Hori ref #209)

Bug fixes

• Fixed a deprecation warning in test_lae.jl (thanks @oyamad , ref #200, #208)

Other

• deleted un-exported and un-used function num_actions from discreteDP code (thanks @a-parida12, ref #207 and #103)
• Some updates in prep for Julia 0.7+ (#213, #214)
• Typo fix (thanks @pkofod #216)