T copter springy legs

This is a collection of springy legs for the "T" Copter series of multi-copters ([T4 quadcopter](http://www.thingiverse.com/thing:261145) and [T6 hexacopter](http://www.thingiverse.com/thing:246350) and the [Tubular Crossfire](http://www.thingiverse.com/thing:261298)). It combines the long and short legs previously included in those various files.
Also featured is a new "split" mounting design that allows the leg to be replaced without removing the arm from the copter (as requested by @khogan