Create schema import/export functions

Example:

from rows.fields import detect_schema, load_schema

table1 = rows.import_from_csv('filename.csv')
schema = detect_schema(table1)

# after
table = rows.import_from_csv('filename.csv', schema=schema)
# or
table = rows.import_from_csv('filename.csv', schema=load_schema('schema.csv'))

Related to #237.

Tasks:

• Create detect_schema function
• Create load_schema function
• Add support for forcing field types in CLI using --schema=filename.csv
• Add schema parameter to create_table
• Decide how to describe custom field classes

Start by reading the existing detect_schema and load_schema functions and the create_table entry point, then review how the CLI currently handles --schema=filename.csv. Done means create_table accepts a schema and the behavior for custom field classes is decided and documented.