Gitter: Errbot should ignore itself

[nemani]

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Issue description

Errbot should not respond to messages made by itself.
This will prevent infinite loop of messages.

Steps to reproduce

Example: !echo !echo !echo hi

[nemani]

CC @meetmangukiya

[gbin]

This is a backend bug, which one are you using ?

[nemani]

The gitter adapter.

[nemani]

I think @meetmangukiya did message on your gitter channel.

[meetmangukiya]

gitter-backend @gbin

[zoni]

I've opened errbotio/err-backend-gitter#12 as this is specific to the gitter backend, not errbot itself.