Added str_error() for better error feedback

str_error() has been provided to give human-readable
error messages after calling get_error(), to translate
between the libzookeeper error constants, and something
meaningful to the end user. This can be removed at such
a time as the C API supports a custom zookeeper-specific
str_error() call.
Merge pull request #1 from geofffranks/str_error

Added str_error() for better error feedback