Change dependencies to require Yesod >= 1.2
Adapting the source code to Yesod 1.2

In particular, Yesod 1.2 introduced significant changes in the handling
of subsites (see
http://www.yesodweb.com/blog/2013/03/big-subsite-rewrite). Due to these
changes and to the GHC stage restriction, we need to declare the routes
in a separate module (Network.Gitit2.Routes).

Note: It does not compile yet, apparently because of a bug in Yesod that
prevent nested subsites from working (see
https://groups.google.com/forum/#!topic/yesodweb/7J4wPwGhwFY)
Replace deprecated types and functions by their corresponding successor.
Move the static subsite upto the master site.

Through not ideal, it is the minimal change to avoid having a nested
subsite inside the Gitit subsite, which does not work with yesod 1.2 (see
https://groups.google.com/d/topic/yesodweb/7J4wPwGhwFY). As a side
effect, we have to change the routes in src/gitit2.hs to avoid
overlapping between the static subsite and the gitit subsite at the
master level.
Remove useless imports in Network/Gitit2.hs
HomeR redirects to front page instead of displaying it.

When displaying the front page from the root of the subsite, the links
in the page are relative to the root url (e.g.  http://localhost:3000/)
instead of the sub-directory of the page (e.g.
http://localhost:3000/wiki).
Generate links with no url relatively to current directory.

It was done this way in Gitit1.
Merge pull request #7 from jmnotin/Yesod-1.2

Port Gitit2 to Yesod 1.2