[SPARK-8434][SQL]Add a "pretty" parameter to the "show" method to display long strings

Sometimes the user may want to show the complete content of cells. Now `sql("set -v").show()` displays:

![screen shot 2015-06-18 at 4 34 51 pm](https://cloud.githubusercontent.com/assets/1000778/8227339/14d3c5ea-15d9-11e5-99b9-f00b7e93beef.png)

The user needs to use something like `sql("set -v").collect().foreach(r => r.toSeq.mkString("\t"))` to show the complete content.

This PR adds a `pretty` parameter to show. If `pretty` is false, `show` won't truncate strings or align cells right.

![screen shot 2015-06-18 at 4 21 44 pm](https://cloud.githubusercontent.com/assets/1000778/8227407/b6f8dcac-15d9-11e5-8219-8079280d76fc.png)

Author: zsxwing <zsxwing@gmail.com>

Closes #6877 from zsxwing/show and squashes the following commits:

22e28e9 [zsxwing] pretty -> truncate
e582628 [zsxwing] Add pretty parameter to the show method in R
a3cd55b [zsxwing] Fix calling showString in R
923cee4 [zsxwing] Add a "pretty" parameter to show to display long strings