Fixed a problem when getSourcePath can contain a directory

When using phiamo/MopaBootstrapBundle and using:
```
{% stylesheets filter="assetdirectory"
    '@MopaBootstrapBundle/Resources/bootstrap/less/bootstrap.less'
{% endstylesheets %}
```
A `[error] File "<dir>/glyphicons-halflings-regular.eot" does not exist` was thrown. 
This was due to `getSourcePath()` containing a directory.

Sorry I don't have time to check/write a test for this at the moment. Will do it later if you want.
Merge pull request #1 from boekkooi/rel-subdir

Fixed a problem when getSourcePath can contain a directory