[SPARK-4205][SQL] DSL expressions with a literal on the left hand side

[culler]

Currently the DSL expression parser does not recognize expressions which begin with a literal, such as:
0 < 'x.

This PR adds a new implicit class which allows such expressions to be included in the DSL. Due to conflicts with scalatest, which converts === and !== in its assert macro, these two operators can not be used by the new class. It adds two new operators -=- and !=- which behave identically to === and !== within the DSL, but can also be used in expressions which start with a literal.

The new class replaces the comment: // TODO more implicit class for literal?
I think this may be what the author had in mind.

[culler]

@marmbrus and @liancheng, here is the other half of PR #3158.

[AmplabJenkins]

Can one of the admins verify this patch?

[marmbrus]

@culler, thanks for working on this. I'm concerned about having too many implicates in scope at any given time and so I'm hesitant to add this (especially as the default set included in the sqlContext). As a result, I propose we close this issue for now and consider some smaller improvements to the DSL.