template: add page_base.html

* Renames page.html to page_base.html and creates new page.html
  to allow overriding of main page template on a Invenio instance.
Merge remote-tracking branch 'jmartinm-github/pu-base-template' into pu
install: new pip install procedure

* To install Invenio run:
  > pip install --index-url=http://invenio-software.org/download/python/simple/ -r requirements.txt

 For extra features run:
  > pip install --index-url=http://invenio-software.org/download/python/simple/ -e .[extras]

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>