Absolute template paths and other fixes

These changes are required in order to test xblock-mentoring for a client.

* Partner information: 3rd party-hosted open edX instance, for an edX solutions client (so will need to get some sort of solution merged in)
* Merge deadline: 2014-10-08 (soft requirement to minimize code drift, we maintain it on the client fork in the meantime)

### Absolute template paths:

Bug: When trying to evoke `manage.py` from outside the repo root, the workbench fails to find the templates.
This is important in several cases, such as selenium integration tests, that are usually ran from outside the repository.

Steps to reporduce (assumed you are in the repo root):
```bash
cd ../; xblock-sdk/manage.py syncdb <<<"no"; xblock-sdk/manage.py runserver;
```

### Bump required XBlock version:

I also bumped the version of the required XBlock to one that has the `enforce_type` fixes.

Without it, there are cases when a `Boolean` `Field` gets overwritten with `'false'` from an xml template, instead of being coerced into a `bool` type.

Please just use os.path, there's no reason to import posixpath directly, is there?