to control the pump, you could easily look for something like this
Only in a 230V version. Basically its a switch that you control with a digital pin from your arduino (trigger it on whenever the humidity gets below like 70%,then after the airs back to 85% or whatever humidity you want, trigger it off to turn off the pump)
Something like that would be extremely easy to use, but after peeking around for a little (i see your from portugal) I'm not finding a control box as easy to use as the 120V one I linked above.
Your other way to control your pump could be to get a nice 230V relay off ebay something about this.
http://www.ebay.com/itm/1-New-25A-SSR-Solid-State-relay-DC-Control-AC-DC3-32V-to-AC24-480V-/120920218074?pt=LH_DefaultDomain_0&hash=item1c276819da
Those are fairly easy to use. Basically you take an extension chord. Cut 1 of the 2 conductors, attaching each end of the now cut conductor to 1 of the 2 "output" terminals on that relay (make sure to do a nice hook around the screw). Then plug the remaining end into the wall (probably have someone electrically inclined to check your cable for safety).
Then connect an arduino digital pin to the + on the control side. and connect the - to the arduinos ground. now when you trigger the arduino pin it'll turn the pump on or off (assumign the pump is plugged into this modified extension chord)
Thatll let you control the misting. then you just need a temp sensor and your humidity sensor