In get_string, end is eval before sortby

```
What steps will reproduce the problem?
1. table = PrettyTable(["Count", "Data"])
2. add rows
3. print table.get_string(sortby="Count", reversesort=True, end=limit)

What is the expected output? What do you see instead?
Expected: Table should be sorted, then decide where to end
Actual: Table is grabbing the end index, then sorting (only beginning of table
to end limit provided is being sorted)

What version of the product are you using? On what operating system?
prettytable 0.7.2
Python 2.7.8
osx 10.9.4

Please provide any additional information below.

```

Original issue reported on code.google.com by `gr...@pagerduty.com` on 24 Sep 2014 at 8:10

Start by locating the get_string implementation and reproduce the issue with sortby="Count", reversesort=True, and end=limit using the steps in the report. Done means the complete table is sorted before the end limit is applied, with the expected rows returned.

Written by the indexing model from the issue text.