Prefer using redis-py's exception classes over our own.

Our own tests run into this issue - when using --use-redis we expected a mockredis exception, but got a redis-py exception.
Merge pull request #73 from mikekap/feature/use_original_exceptions

Prefer using redis-py's exception classes over our own.
Fix the return values of HMSET and HSET
Merge pull request #74 from mikekap/feature/hash_returns

Fix the return values of HMSET and HSET
Merge remote-tracking branch 'upstream/develop' into feature/byte_strings_everywhere

* upstream/develop:
  Fix the return values of HMSET and HSET
  Prefer using redis-py's exception classes over our own.

Conflicts:
	mockredis/client.py
	mockredis/tests/test_hash.py