sql: map fixlen strings to sqlalchemy.String
sql: properly translate string encoding and length

between datashape and sqlalchemy
dshape_to_sqlalchemy uses Text for varlen strings
sql: floating point precision passed through
sql: use mantissa for float precision, not bitlength
Merge branch mrocklin-sql-string-types into feature/csv_to_sql_natively
Proper handling of when csv.header is None