isModuleInstalled added to BackendModel
Changed name to isInstalledModule()

Conform functions like BackendAuthentication::isAllowedModule() and
isAllowedAction().
Removed BackendExtensionsModel::isModuleInstalled()
Replaced BackendExtensionsModel::isModuleInstalled() by BackendModel::isInstalledModule()
renamed isInstalledModule to isModuleInstalled
Router: validate if the installer exists.

Fixes #398
Merge branch 'master' of github.com:forkcms/forkcms
Exit after redirecting to prevent to entire application from being executed.
Bugfix: do not add Spoon headers to the Response.

Spoon has already send the headers. Adding them to the Response object will
cause them to be send again. This caused all headers to be added twice and in
the event of a redirect caused the browser to halt because of duplicate Location
headers.

We should refactor out setting headers via Spoon, this is already being discussed in issue #402.
Update the changelog with recent fixes.
Remove deprecated function
fixed login sql error on wrong email
Location widget address fixed
Removed wrong line from Frontend Pagination

Apparently something went wrong on previous pull-request.
Changelog updated
Merge pull request #431 from siesqo/frontend-linefix

Removed wrong (unneeded) line from Frontend Pagination
updated code after feedback
update html5 shiv
deleteThumbnails function added
beter message