- Dialects can now generate label names of adjustable length.

- Dialects can now generate label names of adjustable length.
Pass in the argument "label_length=<value>" to create_engine()
to adjust how many characters max will be present in dynamically
generated column labels, i.e. "somecolumn AS somelabel".  Any
value less than 6 will result in a label of minimal size,
consiting of an underscore and a numeric counter.
The compiler uses the value of dialect.max_identifier_length
as a default. [ticket:1211]
- removed ANON_NAME regular expression, using string patterns now
- _generated_label() unicode subclass is used to indicate generated names
which are subject to truncation