Deadly Reentry version 6.4.0

Deadly Reentry version 6.4.0 release (KSP 0.90)

*Added toolbar button (for stock toolbar)
*Added Easy, Normal and Hard difficulty settings accessible from new menu
*Difficulty settings are per-save game!!! Use Easy for sandbox and Hard for Career! (if you want)
*Alternate lower density calculation (for use with Hard mode to prevent excessive heating for high speed aircraft)
*Fix for stuttering AeroFX cases. (thanks to Motokid600, Chezburgar7300, Zeenobit and Noio for feedback and/or testing)
*Reworked warning messages for visibility and/or optimization
*Optimized density calculations (moved all into ReentryPhysics; no more per-part calculations)
*Lowered part max-temperature cap to 1250 (other parts may be even lower)
*Heat shields now insulate attached parts against conducted heat
*Low grade heat shielding added to nose cones and fairings. (also to parachute 'cone' parts. Deploying chutes 'jettisons' the shield)
*Kerbals now react to reentry events such as overheating. (may need tweaking; even Jebediah gets scared now. Can't have that)
*Merged in fixes from NathanKell for FAR detection
*Merged in changes from NathanKell to support R&D / Technology requirements
*Added support for (currently unused)stock KSP airstream shielding
*Trapping and checking for of null reference errors in events.
*Added DeadlyReentry.version for AVC and CKAN support