added explicit bind parameters and column type maps to text type

added explicit bind parameters and column type maps to text type
text type also parses :<string> into bind param objects
bind parameters convert their incoming type using engine.type_descriptor() methods
types.adapt_type() adjusted to not do extra work with incoming types, since the bind
param change will cause it to be called a lot more
added tests to new text type stuff, bind params, fixed some type tests
added basic docs for using text with binde params