Use THEME_STATIC_DIR instead of hardcoded 'theme'

The output folder for the theme can be changed via `THEME_STATIC_DIR` setting. But the assets plugin uses the harcoded `theme` string. This small patch makes the asset plugin use the setting.
Merge pull request #107 from graffic/patch-1

Use THEME_STATIC_DIR instead of hardcoded 'theme'
The `asset_url` needs to use also `theme_static_dir`

Sorry for this mistake, I was using harcoded paths in my template.

If the destination changes in the output folder (`THEME_STATIC_DIR`) also the URL used for the assets should change.
Merge pull request #108 from graffic/patch-2

The `asset_url` was also hardcoded instead of using `theme_static_dir`
DOC: nbconvert no longer necessary
Fix error message for IPython 2.0
Merge pull request #110 from jseabold/ipython-fixes

IPython cleanup