* Added new SafeFile (io.WriteCloser) class + methods to atomically write a file.
* Teach core lib to use SafeFile for all file I/O
* Teach sigtool to use SafeFile for all file I/O
* Cleaned up the public interfaces of sign/ to be more coherent:
- with uniform APIs for marshaling, unmarshaling, serialization.
- removed KeyPair class/interface and stick to PrivateKey as the primary
interface.
* collected common rand utility functions into rand.go
* Teach sigtool to NOT overwrite existing output files (keys, signatures etc.)
* Teach sigtool to use a new --overwrite option for every command that creates
files (generate, sign, encrypt, decrypt)
* encrypt/decrypt will try to use the input file mode/perm where possible
(unless input is stdin).
* Added more tests
