Causeless DecimalField behavior

[zzzsochi]

DecimalField should not use float to store its values. It is crucial to never use float for Decimal as it is often used for financial tasks.

I suggest to use Int and precision metainfo somewhere nearby.

[rozza]

DecimalField was a contributed field - I'm more than happy to take a PR to improve it

[lafrech]

FWIW, a new Decimal type was added in MongoDB 3.3.8 / pyMongo 3.4.

Nov 29, 2016: Newly released Mongo DB 3.4 adds decimal data type. It would make sense to use it when using Mongo DB 3.4.

[julianwachholz]

@lafrech any plans for implementing this native NumberDecimal type?

[kotofos]

I highly suggest to add big warning to docs about Decimal - Float conversion.

[denex]

As far as now it completely loses precision I would suggest raising NotImplementedException in order to prevent data loss.

[erdenezul]

Explanation merged into master