Update install.rst

Minor typo
Handle empty subjects properly.

fix #586
Merge pull request #584 from rdpse/patch-1

Update install.rst
Merge branch '1.1.x'
Regroup all queries in one place.

see #589
Fixing demo fixtures.
Merge branch '1.1.x'
Do not override the default history object.

fix #588
Merge branch '1.1.x'
Few fixes in the postgres backend.

see #589
Adding some comments.

see #589
Merge branch '1.1.x' into refactoring/amavis_queries
Merge branch '1.1.x'
Delete the old bootstrap.css and replace by the new 3.2 bootstrap.css
Replace the old glyphicons files by the new ones
Delete bootstrap-responsive.css which will be useless
Rename the folder img in fonts at url modoboa/static
Change html tags in the top navbar and footer navbar
Change html tags in the core of the page
Subsitution of "icon" by "glyphicon" in file admin_tags.py