Added basic theming support (theme/default/posts.php)
Added parsedown as a submodule
Added configuration options for theme and posts path (because of problems with post path showing the directory content on some apache servers)
Changed theming to split the parts of the page in different files, this makes easier theme editing possible

NOTE: Possibly moving the theme/default/posts.php back to app/pages/posts.php but thereby only making possible to modify sections on the theme.
Added proper error handling on non existing post.
Merge pull request #5 from lumius/master

several features adding and fixing one bug

- Added basic theming support
- Wrote a workaround for a problem between url rewriting and folder structures.
- Created a dependency on parsedown.