#Add support for :categories in permalink. & #Fix double-configuration bug ...

Two things are addressed in this commit:

#1. Support for :categories in permalink. First of all no string replacing was happening for categories and also the resolution of categories and tags (in CreatePage) was happening after the resolution of permalink. -- which is too late so moved to before that

#2. In siteContextDrop the code to ToHash was always adding to dictionary while it is likely that a key is present both in the "config" and the "context" itself.

Note: I noticed these issues while trying to get the "jekyll bootstrap template" building with this tool. -- template can be found here: https://github.com/plusjade/jekyll-bootstrap/