Ship rebalancing for hungry/economy autopilots

This is third and last part of issue #1588 being split. Here the ships should be rebalanced in such way, that the hungry and economy autopilots make sense. The changes in C++ part only contain new buttons in UI to call hungry/economy autopilots and changes in ShipType.cpp, which enable to specify fuel usage either by thruster_fuel_use variable, which is now in master, or by effective_exhaust_velocity, which has many advantages. But the moder can pick what suits him/her better.

How the ships are balanced is shown in this <a href=https://docs.google.com/spreadsheet/ccc?key=0Avf9Om_zOYZPdFVDSUVZZmk5U0M0elFRSXlNQ1d3V1E#gid=0>excel sheet</a>. In general, these ideas were followed:
* Fuel use rate should be higher so there is difference between hungry and economy autopilot.
* Effective exhaust velocity (v_c) should be similar for all ships, since it depends only on engine technology (for example chemical rocket engines have v_c ~ 2.5 km/s, ion thrusters 29 km/s to 250 km/s depending on how they are made). Currently, smaller ships have slightly higher v_c since they probably use faster engines to reduce 
* Ship's mass was increased by 30% and the mass was added to fuel tank for smaller ships and to cargo space for bigger ships. This improved the travel speeds of (particularly the smaller) ships.
* Accelerations should be always smaller than 20 G (maybe even 15 G) so that the pilots could survive it.
* Bigger ships (freighters) should have smaller acceleration.
* Minimum of fwd, rev, up, down, left, right thrusts should be such that the acceleration of laden ships is still big enough for landing at planets (unless we specifically want ship incapable of landing.) 

If you have any other issue to be considered, please post in into the discussion!