Add sqlite build tag

This adds a sqlite build tag so that you don't have to have the sqlite
import commented out in code and users can run:
    go build -tags sqlite
if they want to have sqlite support enabled.  It is disabled by default
so nothing changes with the default go get or build commands.
Merge pull request #75 from crosbymichael/sqlite-build-tag

Add sqlite build tag
Merge branch 'master' of github.com:gogits/gogs into dev
Merge branch 'dev' of github.com:gogits/gogs into dev