Merge pull request #379 from olasundell/master

The Makefile is now OSX and Linux aware
Revert "Merge pull request #379 from olasundell/master"

This reverts commit 5ec216800146df0503685e544210ee1daef8d2a1, reversing
changes made to db65f55284a7e45e586defa1dbe280d5f6f2e37d.
Releasing 2.3.0
Merge pull request #398 from MuMech/dev

MechJeb 2.3.1 release
Merge pull request #399 from MuMech/dev

Removed revision checking from CompatibilityChecker, In case we get a KSP 0.24.3 >.>
Maneuverplanner modularization

Maneuvers are now implemented in separate classes.
Each class draws its GUI and has a method to compute the node.
Parameters are inside the Operation object and are not shared anymore.
Implement a (new) combobox utility

Switch maneuver planner selection to the new combobox
Fix steam directory under Linux
Revert "Fix steam directory under Linux"

This reverts commit 8fad790541b2f8af39f913774701e34a2c5ead85.