[MRG] Edits to the introduction. (#129)
[MRG] Edits to the introduction. (#129)

* Minor edits to the introduction.

* Continuing to clarify the introduction.

* Note that the last sentence in the `Initialization` section
  was completely removed. It was unclear what this sentence was
  meant to say, but did not make sense in the context of the
  surrounding paragraph.

* Edits to `Evolution` section.

* Some additional explanation is necessary in this section.
  Specifically, does the winner of a tournament undergo exactly
  one of these operations, or does it undergo potentially all of
  these ops, with the probability of each op being performed given
  by its respective parameter?

* Small changes to last few sections of introduction.

trevorstephens/gplearn