KSP 0.90 update (DRE beta 6.3.* to DRE 6.4.0)

*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