bump version

* add some doc about wtf this thing does
* pin Twisted to < 18.4
* add explicit dep on six (fixes #3089)

* When creating a new event, cap its depth to 2^63 - 1
* When receiving events, reject any without a sensible depth

As per https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI

Changes in synapse v0.28.0-rc1 (2018-04-26)
===========================================

Bug Fixes:

* Fix quarantine media admin API and search reindex (PR #3130)
* Fix media admin APIs (PR #3134)

Changes in synapse v0.28.0-rc1 (2018-04-24)
===========================================

Minor performance improvement to federation sending and bug fixes.

(Note: This release does not include state resolutions discussed in matrix live)

Features:

* Add metrics for event processing lag (PR #3090)
* Add metrics for ResponseCache (PR #3092)

Changes:

* Synapse on PyPy (PR #2760) Thanks to @Valodim!
* move handling of auto_join_rooms to RegisterHandler (PR #2996) Thanks to @krombel!
* Improve handling of SRV records for federation connections (PR #3016) Thanks to @silkeh!
* Document the behaviour of ResponseCache (PR #3059)
* Preparation for py3 (PR #3061, #3073, #3074, #3075, #3103, #3104, #3106, #3107, #3109, #3110) Thanks to @NotAFile!
* update prometheus dashboard to use new metric names (PR #3069) Thanks to @krombel!
* use python3-compatible prints (PR #3074) Thanks to @NotAFile!
* Send federation events concurrently (PR #3078)
* Limit concurrent event sends for a room (PR #3079)
* Improve R30 stat definition (PR #3086)
* Send events to ASes concurrently (PR #3088)
* Refactor ResponseCache usage (PR #3093)
* Clarify that SRV may not point to a CNAME (PR #3100) Thanks to @silkeh!
* Use str(e) instead of e.message (PR #3103) Thanks to @NotAFile!
* Use six.itervalues in some places (PR #3106) Thanks to @NotAFile!
* Refactor store.have_events (PR #3117)

Bug Fixes:

* Return 401 for invalid access_token on logout (PR #2938) Thanks to @dklug!
* Return a 404 rather than a 500 on rejoining empty rooms (PR #3080)
* fix federation_domain_whitelist (PR #3099)
* Avoid creating events with huge numbers of prev_events (PR #3113)
* Reject events which have lots of prev_events (PR #3118)

Fix media admin APIs

Fix quarantine media admin API

Rephrase v0.28.0-rc1 summary

fix formatting on line break

This reverts commit 08b29d45.

Reject events which have lots of prev_events

Make tests py3 compatible

Use six.itervalues in some places

add __bool__ alias to __nonzero__ methods

Add some more variables to the unittest config

Replace Queue with six.moves.queue

Use str(e) instead of e.message

Clarify that SRV may not point to a CNAME

Document contrib directory