Fixed build, added support for negative indexes in Robot keywords

This pull request fixes the rtomac:master build and adds support for negative indexes in Robot keywords.

# Fixed Se2Lib build

@emanlove, this fixes the build failure currently noted at https://github.com/rtomac/robotframework-selenium2library.

Passing Travis CI build:

- https://travis-ci.org/rtomac/robotframework-selenium2library/jobs/33288022

The 14 critical test failures reported in the current rtomac:master build (https://travis-ci.org/rtomac/robotframework-selenium2library/jobs/23423255) are caused by unexpected content in test/results/output.xml.  An extra line was added in the log output of a frequently-used command -- likely due to an update in the upstream Selenium 2 package.  

The failing Se2Lib tests simply needed to account for the fact that the expected log file content is now a few lines ahead of where it was previously.

I also applied a fix for the 'Choose File' bug by @HelioGuilherme66 (from 532295cbce).

# Support for negative indexes in Robot keywords

Robot keywords like `Table Row Should Contain` support looking up elements with a positional, 1-based index, e.g.

    Table Row Should Contain   ${table_locator}    1    ${expected_first_row}

verifies that the first row (row 1) in the table with the ID defined in 'table_locator' has the expected content defined in 'expected_first_row'.

Unfortunately, `Table Row Should Contain` does not currently support easily looking up content from the end of a table. For example, it would be very helpful to be able to write something like

    Table Row Should Contain   ${table_locator}    -1    ${expected_last_row}

to verify the content of the last row (row -1) in a table.  Row -2 would be the second-to-last row, and so on -- just like negative indexes in Python lists.

This pull request adds support for negative indexes to the following Robot keywords:

* `Table Row Should Contain`
* `Table Column Should Contain`
* `Table Cell Should Contain`