Update security.md

Fix typo at line 81, missing ')' at the end of if clause.
Changing order of installation options
Update installation.md
Changing installation order
Updating docs with associate method.
Added framework sponsors.
Update documentation.
Fix code sample for associating models.
Merge pull request #302 from franzliedke/patch-5

Fix code sample for associating models.
Clarify documentation on date mutators.
Fix sponsors list

The list was being converted to a code block because of the tabs.
Merge pull request #303 from driesvints/patch-3

Fix sponsors list
Merge pull request #299 from AustinW/master

Fixed the order of the Eloquent content sections to match the table of contents
Document manually logging in users. Closes #295.
Document firstOrFail.
Merge pull request #294 from AoSiX/master

Changing installation options order
Merge pull request #293 from andrefjpinto/patch-1

Update security.md
Add note about unsigned on foreign keys. Closes #298.
Add helper documentation.
Fix typo.