sources: Pass any current mode to `_populateMetadata` when finding pages. Page sources like the `prose` source may need to open an existing page's file to read stuff from it. This won't work if the metadata is populated as part of finding a path to create a page (like when running `chef prepare`). We pass the mode to `_populateMetadata` so the underlying class now knows the current context in which it is called.

Jinja2==2.7.3
Markdown==2.4.1
MarkupSafe==0.23
PyYAML==3.11
Pygments==1.6
Werkzeug==0.9.6
colorama==0.3.1
compressinja==0.0.2
mock==1.0.1
py==1.4.23
pytest==2.6.1
pytest-mock==0.2.0
python-dateutil==2.2
repoze.lru==0.6
six==1.7.3
smartypants==1.8.6
strict-rfc3339==0.4
textile==2.2.1