SchemaGenerator, ContentResolver: read floats as BigDecimals

ObjectMapper defaults to using double for floats. But many applications require
the precision of BigDecimals.

Use the corresponding feature in all ObjectMappers.
SchemaMapper: use a JsonNodeFactory to create ObjectNodes

No need for a full mapper there.

What is more, a JsonNodeFactory is thread safe: use only one, statically
initialized instance.
Merge pull request #1 from fge/master