Merge pull request #6 from Demestotenes/code-structure

Extensions loading
ignore extensions returning None
invenio.ext.jinja2hacks
moved jinja2hacks.py
jinja2utils -> invenio.ext.jinja2, invenio.ext.jinja2hacks
jinja2utils done
signalutils -> base.signals
adminutils -> ext.admin
invenio.hashutils -> invenio.utils.hash
utils in setup.py
adminutils fixes