introspection fails in postgres when database contains table with same name in two different schemas
Thank you for this issue, looking into it now.

I tried to introspector.metadata.get_columns('isrc') and got following error:

```
C:\Python27\lib\site-packages\playhouse\reflection.pyc in get_columns(self, table)
    230             columns[name] = Column(
    231                 name,
--> 232                 field_class=column_info['field_class'],
    233                 raw_column_type=column_info['raw_column_type'],
    234                 nullable=column_info['nullable'],

KeyError: 'field_class'
```

To duplicate error create PostgreSQL database 'testdatabase' with two schemas 'public' and 'music' and table 'isrc' with different columns in both schemas. Then run:

``` python
from peewee import *
from playhouse.reflection import Introspector

db = PostgresqlDatabase('testdatabase')
introspector = Introspector.from_database(db)
introspector.metadata.get_columns('isrc')
```

Reson for failure seems to be query from lines 192-198 

``` python
    def get_columns(self, table):
        # Get basic metadata about columns.
        cursor = self.execute("""
            SELECT
                column_name, is_nullable, data_type, character_maximum_length
            FROM information_schema.columns
            WHERE table_name=%s""", table)
```

It returns table columns from all schemas. I think that It should return only columns related on current schema...
