po2prop: expand test

Expand the test to two units to try replicate issue in bug 115.
fn tests: po2prop using bug 115 example

Created a functional test for po2prop using the example from bug 115.
This was not an issue it seems but we now have a test for --personality.
po2dtd: improve test readability
po2prop: add missing accesskey test
po2prop: use the source language accesskey if translations is missing

Fixes bug 115 comment 6
Remove unnecessary comment in requirements
Adjust comment in requirements
Cleanup: We already require lxml >= 2.1.0

So these checks are unnecessary.
Bug 3194 - Faulty error message while initializing properties storage

Just improving error message when unable to find encoding for a string.
Fixes #3194.
Minor cleanup
styleguide: Clean up two trailing spaces
styleguide: Add a clause regarding string formatting
Fix conversion of successive Gaia plural units

When a plural unit is followed by another one, we were failing to output
the first one. Another repeated code block, but that is for the next
commit.
Refactor code repetitions
Add Gaia roundtrip script

Based on roundtrip-mozilla one.
dtd: Use source accesskey if target accesskey is missing
dtd: don't remove stray & as they probably &amp;

Since we now convert & -> &amp; we no longer need to worry about these busted
ampersands.
accesskeys: don't use space as an accesskey
PEP8: E101 indentation contains mixed spaces and tabs
typo