Expand default.style documentation

These changes are purely in comments and the unused nocache flag

Fixes #66

Comments from earlier feedback, which I've hopefully integrated.

__hstore needs a general explanation of how it interacts with the `default.style`__
```
08:19 < apmon> pnorman: I had a look at the default.style docu
08:19 < apmon> looks like a good improvement
08:19 < apmon> a couple of comments though:
08:19 < apmon> I think the phstore flag needs some additional explanation of how it works
08:20 < apmon> The delete flag explanation should probably mention that it also
    deletes it from --hstore-all
08:22 < apmon> I wonder if the section on osm_user, osm_uid also should have a
    mention, that if you import with hstore, then those will be in the tags 
    column independent of if they are listed here
08:23 < apmon> coming to think of it, perhaps hstore needs a general 
    explanation of how it interacts with the default.style
08:23 < apmon> i.e. how --hstore-all --hstore and --hstore-match-only are 
    effected by default.style
```

I must admit I don't fully understand the interaction between `--hstore`, `--hstore-match-only`, `--hstore-all`, `--extra-attributes` and `default.style` and I doubt the average user does.