This reverts commit 780ffa98.

This reverts commit bd7891f2.

I was running this simple condition:
  boobank history XXX --condition 'amount > 2000'
Before this patch, the command raised an error:
  Field "amount " is not valid.
Thanks to this patch, condition are less confusing for users. A blank is
not significant anymore.

The user may now use conditions on durations (from the audio and the video
capabilities) in repl applications.

The documentation has been updated to take this change into account.

Actually, the condition are not widely used. Do not load the datetime
module if we don't run it.

And use obj.to_dict instead of dict(obj.iter_fields())

And return a better message to users

closes #1308

It is now possible to get a history in a range of date with something
like:
boobank history CC-1234@ing --condition "date>2013-12-01 AND date<2013-12-8"

It displays transactions done betwenn the 2th and the 7th december.

Support < and > operators
Simplify some code

Allow to give int, float, and not only strings
Fix AND/OR conditions
Add some comments to unterstand this fucking code

Return True at the first True result, return False if no condition
match

Closes #1288