Create schema when 'creating all' if it doesn't exist
I am going to pass on this, but thanks for the patch.
As it says in the table. Not sure if we want to use the safe version of this method in `_create_table` itself, which is also possible. Also, I was not sure where to put Unit Tests for this. If you give me a location, I can write them if you want.

PS. One test fails for PostgreSQL, but it also fails without my pull request.

``` python
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FAIL: test_db_table (tests.schema.TestModelDDL)
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Traceback (most recent call last):
  File "/home/karel/working/peewee/tests/schema.py", line 126, in test_db_table
    'CREATE TABLE "A_tbl" ("id" INTEGER NOT NULL PRIMARY KEY)'])
  File "/home/karel/working/peewee/tests/schema.py", line 83, in assertCreateTable
    self.assertEqual([sql] + indexes, expected)
AssertionError: Lists differ: ['CREATE TABLE "A_tbl" ("id" SERIAL NOT NULL PRIMARY KEY)'] != ['CREATE TABLE "A_tbl" ("id" INTEGER NOT NULL PRIMARY KEY)']

First differing element 0:
'CREATE TABLE "A_tbl" ("id" SERIAL NOT NULL PRIMARY KEY)'
'CREATE TABLE "A_tbl" ("id" INTEGER NOT NULL PRIMARY KEY)'

- ['CREATE TABLE "A_tbl" ("id" SERIAL NOT NULL PRIMARY KEY)']
?                              ^  ---

+ ['CREATE TABLE "A_tbl" ("id" INTEGER NOT NULL PRIMARY KEY)']
?                              ^^^^^


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```