Remove streamIV_template.c

The existing stream_template.c can be used for Salsa20 too.
A macro (STREAM_CIPHER_NEEDS_IV) controls if
the "iv" parameter is needed or not.

Declare a variable as const to remove a compiler warning.
Unit tests now pass on Python 3.x
Unit tests now pass on Python 2.1
Mentioned source of test vectors
Merge pull request #1 from Legrandin/salsa20

Salsa20