Drop Python 2.6

[d0ugal]

We should aim to drop Python 2.6, probably not in the next release, but the one after, or before 1.0 at least.

• We have a number of hacks in place to deal with posixpath problems.
• Python Markdown has dropped Python 2.6 support, so we currently can't use the latest version

Doing this will involve:

• Updating docs
• Removing 2.6 test runs
• Removing 2.6 hacks/fixes

[ngzhian]

Afaik, the places where we have fixes for 2.6 are:
1. nav.py#95
2. nav.py#103