Add support for String contatenation through the "+" operator

Since 4.0, Cassandra support operations on numeric types and on some temporal types.
We should add support for string concatenations through the + operator.

Additional information for newcomers:

Cassandra has 2 string types: Text (UTF8Type) and ascii (AsciiType) both are sub-classes of (StringType). In order to add support for string concatenation you will need to add a new method to StringType and modify OperationFcts to add the new functions (one for each combination of types: ascii/ascii, ascii/text, text/ascii and text/text).

The unit test should be added to OperationFctsTest.
To allow for concatenating constants (eg: SELECT v1 + ' ' + v2 FROM ...) you will need to add a getPreferedTypeFor method to org.apache.cassandra.Constants.Type.STRING that determine the constant type based on its content (ascii or not).