Only CMake >= 2.8.3 works. Add explicit requirement + document it.
Update reference to Getkw + change thresholds in Green's function numerical tests.

The bug unearthed by TDC in Getkw should be now fixed. Also the problem with numerics pointed out by LV is fixed:
the threshold for the numerical evaluation of the Green's function derivatives was too tight.
Get fix for libgetkw
Update check_external_timestamp.py script
Merge branch 'master' into release