PolarSSL: update to current stable version (1.3.4)

I just removed Externals/polarssl/, added the new version, then deleted
the following files/directories:

DartConfiguration.tcl
Makefile
doxygen/
library/Makefile
programs/
scripts/
tests/
visualc/
PolarSSL: adapt Dolphin to new version

- strip down PolarSSL's CMakeLists.txt
- switch to the PolarSSL 1.3 API
- use entropy interface instead of havege (PolarSSL 1.3 has disabled
  havege by default because it is "considered unsafe for primary usage")
- add VS2013 .vcxproj file
PolarSSL/CMake: validate cached values
CMake: check PolarSSL version
Merge pull request #102 from Tilka/polarssl1.3

Update PolarSSL to current stable version