Add Peewee integration

[datadog]

Add Peewee integration

[brettlangdon]

From talking with @jontonsoup, they are using PostgreSQL/psycopg2, and have used patch_all, which should have patched psycopg2.connect, but that didn't seem to work.

It appears that peewee is using psycopg2.connect, which should have been patched:

https://github.com/coleifer/peewee/blob/master/peewee.py#L4043

[jontonsoup]

@brettlangdon apologies -- The trace is working, but pinning does not work.

conn = DB.get_conn()
pin = Pin.override(conn, service="user-db")

WARNING:ddtrace.pin:can't pin onto object. skipping
Traceback (most recent call last):
File "/Users/jonathan/src/lib/python3.5/site-packages/ddtrace/pin.py", line 96, in
onto
return setattr(obj, '_datadog_pin', self)
AttributeError: 'psycopg2.extensions.connection' object has no attribute '_datadog_pin'

[brettlangdon]

We have recorded better PeeWee support as a feature request internally, and will plan work to make psycopg more configurable without using a Pin.