Add Instructions to Installation for Development

An introductory paragraph has been added to the "Installing Freeseer
for Development" section, giving an overall summary of how to install
Freeseer for development. It links to the guide on how to fork and
clone the project at the beginning of the section instead of the end.

In addition, the dollar signs $ have been removed from any command
line instructions for consistency and clarity.

Related to #442
Close #599
:memo: Edit copy for installing Freeseer for dev

Make it more concise and put steps in list-form.

[skip ci]
Close #597 Document how to install Freeseer in editable mode

[skip ci]
Fix #575: Add sphinx dependency to dev-requirements.
- Added Sphinx to the dev_requirements.txt file.
- Changed Sphinx version to 1.2.3 to reflect latest version.
- Removed docs/requirements.txt from docs and travis.yml.
Fix #584 unicode string format error, 'string' to u'string'
Fix #583 Chinese Translation problem

Fixes #583.
Presently, the translation rates for Traditional Chinese and Simplified Chinese are 100%
Update About tab of ConfigTool

- Made About tab the last tab in the list
- General tab now the default tab
- Freeseer logo moved to right side
- Modified test for default widget
- About tab layout similar to Recording
Update the testing doc page from unittest to pytest

The contributor testing page contained examples of writing UnitTest
unit tests. The references and examples have been updated with pytest
information instead of UnitTest information. Therefore, new or existing,
contributors will now have a brief introduction to writing and running
the Freeseer pytest unit tests.

Fix #566
Related #420
Related #484