Document Translate Toolkit use of pytest and plugins
Revise pytest docs with code-block & cross-refs

add pytest to intersphinx links in conf.py
refactoring comma etcetera
update pytest version dependency & recommendation
add (non-inline) hyperlink to pytest.org
one combined code-block:: bash for all command-line examples
eliminate Plugins section: importorskip() and deprecated_function() are builtin
rewrite skipping text to reflect above, drop hyperlink for :ref:/:func: below
add subsubsections for each function and decorator
add intersphinx :func: cross-reference for importorskip()
code-block:: python for all examples
use :ref: for skipif - expands to subsection title, so reworded to match
note decorators on classes require Python 2.6
use :ref: for xfail - expands to subsection title, so reworded to match
expand explanation of xfail decorator parameters, fix bug in simple example
intersphinx :func: for deprecated_call(); as it's builtin, now before recwarn
italicized hyperlink to recwarn as it lacks defined :ref: target
Better use of intersphinx :ref:, less passive voice
Document the use of code-block's
Fix some docstring formatting
Drop indent
Switch between highlighters
Changes in PHP format docs
Trim and polish TBX format docs

Removed a lot of TBX standard explanations that can be found in other places.
Minor changes in TBX format docs
comment typos
correctly report libgettextpo version when major != 0
Add framework for more extensive cPO testing

USECPO environment testing
more detailed py.test results
always notify on failure
Force stored encoding for cPO to UTF-8
Revert use of 'with' statement for python 2.4

this commit should be reversed when 2.4 support is dropped
Attempt to fix buildxpi.py

See
https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.l10n/sZrgTqBX6I4
Remove incorrect parameter
Don't test Trados if we don't have BeautifulSoup
Docs: Typos and minor highlights in formats
Docs: Changes in po format

* Rearranged items
* Splitted items
* Added examples
* Removed broken link
* Removed section
* Merged sections