Enable translation of plural forms

Issue #238
Remove TranslationStringFactory

TranslationFacotry is only useful for non-plural forms. It only defines a
default domain for a particular string, and it's easily replacable with
a `setdefault`

https://github.com/Pylons/translationstring/blob/master/translationstring/__init__.py#L131

Issue #238
Update l10n source files
Consistant function names
Automatically pass desired changeset comment to iD editor

NOTE: the iD side of it is done <https://github.com/openstreetmap/iD/issues/2311>
and is not yet live on OSM website, but will be when next iD version
comes out.
Make sure python also escapes any slashes in changeset comment prefill
Merge pull request #297 from dodobas/l10n_plural

Support L10n plural forms
Upgrading Pyramid to 1.5.
Merge pull request #318 from danstowell/topic/commenttoid

Automatically pass desired changeset comment to iD editor
Merge pull request #328 from pgiraud/upgrade_pyramid_1_5

Upgrading Pyramid to 1.5.
Allow users to customize layout
Adding custom files to gitignore
main.css generated file added to git ignores
Adding documentation about customization
Don't ignore custom files
Adding favicon
Merge pull request #324 from pgiraud/custom_layout

Custom layout
Add ability to mention users in tasks comments.
Merge pull request #327 from pgiraud/mention

Add ability to mention users in tasks comments.
Logged in user not available in some templates