Add code for XMPP bots

[rolandgeider]

This includes the code for both the collector and the output bots. Code is a tidied up version of the code from PRs 158 and 197, as well as the comments there.

Thanks to @helderfernandes1279 for the original code.

[sebix]

• The tests are failing because the BOTS-file should be sorted (as the ordering is directly used by the manager)
• What is the minimum version of sleekxmpp necessary (or tested with)? Please add this to the requirements
• As we are still supporting Python2, please add from __future__ import unicode_literals to the collector.
• The output bot class should be named XMPPOutputBot for the sake of consistent naming.
• Please don't import sys from intelmq.lib.bot

[codecov-io]

Current coverage is 66.44%

Merging #462 into master will decrease coverage by -0.32% as of 3705e99

@@            master    #462   diff @@
======================================
  Files          173     175     +2
  Stmts         6682    6750    +68
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           4461    4485    +24
  Partial          0       0       
- Missed        2221    2265    +44

Review entire Coverage Diff as of 3705e99

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[rolandgeider]

@sebix I have fixed the issues in one go, while I'm all for atomic commits, I think in this case it's OK :)

[sebix]

Thanks for this contribution!

[sebix]

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