Make table and db iterable

[noirbizarre]

This PR makes Table iterable and TinyDB iterable over default table so it's possible to write:

# Iter over default table
db = TinyDB()
for obj in db:
   do_something(obj)
table = db.table('my_table')
for obj in table:
   do_something(obj)

[msiemens]

I think that's a nice addition. I'm still thinking about where this should be described in the docs: basic usage or advanced usage.

@eugene-eeo What do you think about this change?

[noirbizarre]

I just added a simple documentation proposal

[eugene-eeo]

@msiemens I'm fine with it since it opotimises for the 80% use case. But it needs to be documented somewhere. Perhaps in Basic Usage would be appropriate.

[msiemens]

since it opotimises for the 80% use case

Good point. I think it's quite intuitive for iter(db) to be an alias for db.all(). Thanks for the Pull Request @noirbizarre!

[msiemens]

Also, I've now released TinyDB v3.3.0 which includes this feature :)