rename auto encoding tests and add tests for c_string_type=unicode

--HG--
rename : tests/run/unicode_ascii_encoding.pyx => tests/run/str_ascii_auto_encoding.pyx
rename : tests/run/unicode_default_encoding.pyx => tests/run/str_default_auto_encoding.pyx
rename : tests/run/unicode_ascii_encoding.pyx => tests/run/unicode_ascii_auto_encoding.pyx
rename : tests/run/unicode_default_encoding.pyx => tests/run/unicode_default_auto_encoding.pyx
document c_string_type and c_string_encoding directives in string tutorial