Made the readme consistent with the setup.py

[gabrieldemarmiesse]

Fixes #513 since the decision was taken to make pymongo a mandatory dependency.

[coveralls]

Coverage remained the same at 85.08% when pulling 73dc588 on gabrieldemarmiesse:fix_readme into 4b4158b on IDSIA:master.

[JarnoRFB]

I think it is an error of mine that pymongo is no longer optional. I need to investigate this.

[JarnoRFB]

It seems like that in other observers, the optional imports are scoped differently, so they only get imported once the observer get executed. For the MongoObserver, pymongo is imported toplevel. However, I cannot find any point in the file history where this was changed recently, so I am a bit confused why pymongo is now no longer optional, or rather why it was optional before. Should we scope pymongo as well to make it optional again or should we now just make it part of sacred as it is probably the most used observer anyway?

[JarnoRFB]

ok, I found it now. I try to revert it

[gabrieldemarmiesse]

You can use github's git blame on the setup.py to get the PR which introduced pymongo as a dependency. (if you already know about it feel free to ignore this).

[gabrieldemarmiesse]

closing this PR as it cannot be merged without being changed significantly.