Add return_null option for partition

[soyking]

As described in #26 #43, use RETURN NULL could accelerate the insert action. So here I add the return_null option when use architect.install. It will create the before-insert-trigger without return value and won't create the after-insert-trigger in PostgreSQL.

But there are many limitations:

• it only works on PostgreSQL
• for orms like django pony(psycopg2) sqlalchemy(psycopg2) and sqlobject(psycopg2), user should maintain the model's primary id as I did in tests. In some situations, such as using uuid as the primary key.
• peewee of current version would always fetch the primary key, so it seems not possible to use it as the author said in this issue

It's ok if the MR won't be merged since there are too many limitations which should be declare cleary in documents. Actually I want to know if it could pass the tests in travis :)

[soyking]

hmm… werid error, seems irrelevant with the changes

[maxtepkeev]

I restarted the job and tests are passing now. You're right, there're some limitations which I need to think about before considering whether to merge this or not. Anyway great work, really appreciate it. Let's keep this PR open for a while.