Adds RichDate and RichTimestamp classes with comparison operators,
allowing them to be used in DSL expressions.  These classes provide
initializers which accept string representations of dates or times.
They are renamed as Date and Timestamp when the members of an
SQLContext are in scope.
Adds new implicit conversions which allow DSL expressions to start
with a literal, e.g. 0 < 'x .
These conversions expose a conflict with the scalatest === operator
if assert(X === Y) is used when the conversions are in scope.  To
fix this, several tests are modified, as recommended in the scalatest
documentation, by making the change:
assert(X === Y) --> assert(convertToEqualizer(X).===(Y))