[WIP] Added propel init command

![bildschirmfoto 2014-08-04 um 19 02 31](https://cloud.githubusercontent.com/assets/2072185/3800496/26f1a7ea-1bf9-11e4-95b6-d89b62169735.png)

Based on the discussion in #679.
This is currently work in progress but I want to get some early feedback.

Still to do:
- [ ] Check if we can connect to the db with the credentials provided by the user
- [ ] Allow running reverse-task on existing databases via init command (suggested by @BunnyHolder)
- [ ] Add DSN generation for all rdbms's (currently only mysql)
- [ ] Add prod/dev context to generated `propel.yml` (we should probably generate a `propel.yml.dist` instead of a `propel.yml`, or we could use env-vars for passing the credentials, not sure on this)

So what do you think of this? Let me know if you have some suggestions :)
This requires a writable check