copy RedisDB::Error as RedisDB::Parser::Error

so I could remove dependency on RedisDB. Also add error_class attribute
to the parser, so RedisDB could set it to RedisDB::Error and use this
parser without breaking stuff that still expects errors to be of
RedisDB::Error class.
remove RedisDB dependency
bump version 2.19_03
document error_class attribute
load error class in XS parser
add no-leak test
bump version 2.19_04
check for warnings
add BUILD_REQUIRES to Makefile.PL
add test for utf8 option
bump version v2.19_05
extend SEE ALSO section
document parsing results