New illustrations
* change feeds
* dates and times
* sql to reql cheatsheet
* securing your cluster

Also added the table-joins image to the eq_join api page
Updated secure-cluster and changefeeds imgs
Added sharding_and_replication image
Adding info about key length limitations
Change formatting of generated Python docs
Merge branch 'issue-234-gen-python'
Fixes #234
Updating based on review
Merge branch 'issue-430-key-lengths'; fixes #430
Update pip install command

The old command failed to install the dependency. Running the command returned error:
```
Downloading/unpacking version==1.12.0-2
Could not find a version that satisfies the requirement version==1.12.0-2 (from versions: 0.1.0, 0.1.1)
Cleaning up...
No distributions matching the version for version==1.12.0-2
```
When asked about the issue via IRC @atnnn told me that the error is not OS dependent, so therefore I deiced to file a pull request for it.
Fix link to to_array
Merge pull request #451 from rethinkdb/michel_broken_link

Fix link to to_array
Updated shard-and-replicate image to 270px
Add limitations page; fixes #9
Merge branch 'issue-9-limitation-page'
Merge pull request #431 from rethinkdb/josh/illustrations

New illustrations
Resized dates-and-times.png to 200px
Merge pull request #456 from rethinkdb/josh/datetime_resize

Resized dates-and-times.png to 200px
Add "command syntax" header
Merge pull request #458 from rethinkdb/issue-454-eqjoin-header

Add "command syntax" header
Updating MongoDB comparison; fixes #450