Updates polymorphic relation example

I believe this was meant to be morphMany not hasMany.
added example of attaching view composer to multiple views at once
Adding docs for DB::statement()
Add documentation for date validation rules
Applying callback to the objects in a collection

I love this and I think this should be in the documentation too.
Update events.md
Update installation.md

Added a note that the install process requires Git on the server to successfully complete.
Merge pull request #98 from eoconnell/patch-3

Updates polymorphic relation example
Merge pull request #127 from mikhaill/patch-1

Update installation.md
Merge pull request #116 from raftalks/patch-2

Applying callback to the objects in a collection
Merge pull request #110 from akuzemchak/sql-statements

Adding docs for DB::statement()
Merge pull request #101 from xsbeats/view_composer_array

added example of attaching view composer to multiple views at once
Merge pull request #109 from alexwhitman/date-validation-rules

Add documentation for date validation rules
update the events docs.
update the seeding docs.
update mutator docs.
Merge branch 'master' of github.com:laravel/docs