Remove sqlite + reorganize the code in packages

- SQLite was removed, that required (closes #23 & #24):
  + unique tags & categories managing
  + sort by dates
  + kinda querying for categories, page, tags...
  + blablabla (check the full patchset for that)

- we have a Context struct now that is the one that is going to be sent
  to the templates. It usually gets all it needs, but if not, we have as
  well some template functions to help it.

- the slugs are an absolute path now. If we use metadata to set them,
  .html and / is going to be added (in case that doesn't have it yet).

- splited through several packages.

- fixed tests after all those changes.

- golint passed, except for the magically generated bindata.go.

- I've moved a lot of stuff (I/O bounding) to go routines inside waiting
  groups.