added lazy sender initializer

[mastak]

These changes provide the lazy initializer for the sender instance. It can help with issue with forked scripts like uwsgi:
http://uwsgi-docs.readthedocs.io/en/latest/ThingsToKnow.html

uWSGI tries to (ab)use the Copy On Write semantics of the fork() call whenever possible.
By default it will fork after having loaded your applications to share as much of their memory as possible.

So we should create Queue after fork.

PS Also, I faced this issue in celery and this fix helps me.

[coveralls]

Coverage remained the same at 100.0% when pulling 6e51a47 on mastak:feature/lazy-sender into 268f577 on fluent:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 6e51a47 on mastak:feature/lazy-sender into 268f577 on fluent:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 6e51a47 on mastak:feature/lazy-sender into 268f577 on fluent:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 4ebb946 on mastak:feature/lazy-sender into 268f577 on fluent:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 6e51a47 on mastak:feature/lazy-sender into 268f577 on fluent:master.

[arcivanov]

Looks reasonable. Thanks!