updated README for consistency

The documentation section has all sentences ending with a period except for number 3. I've added the period to make the section more consistent with the rest of the document.
Prevent 'cannot read' exception

On Android (4.4.4) I'm getting "Cannot read property 'options' of null" each time, 100% reproducible.
This small check will not hurt anyone but will prevent unhandled exception error.
chore(jscs): add gulp task for jscs
chore(package.json): sort devDependencies
fix(scroll): $historyId should default to root
chore(update): angular v1.3.3
Merge pull request #2536 from hzoo/add-jscs-task

chore(jscs): add gulp task for jscs
fix(scroll): prevent 'cannot read' exception
fix(scroll): prevent 'cannot read' exception
Merge pull request #2295 from richguan/patch-1

docs(readme): updated README for consistency
refactor(backButton): separate show/enable logic

Previously the showBack property was setting if the back button should
or should not be enabled, and it was also used if the back button
should be hidden or not for the view. Changed it so there are now two
concepts, where showBack is visual only, and enableBack determines if
it should show according to $ionicHistory and navigation info.
fix(backButton): add .header-item class

Add the .header-item class to each of the root child items of a nav
header bar. This gives CSS more power to state what should and should
not be shown during the different states of a transition, specifically
for iOS nav bar transitions.
Update menuToggle.js

Matched parent element to description in usage.
Merge pull request #2581 from tlaverdure/master

docs(menuToggle): update example
chore(slideBox): speed up velocity a bit
docs(ionSlideBox): update example
fix(ionicons): animated icons may rotate on older android devices
refactor(navViewController): move view switcher logic

Move some of the $ionicViewSwitcher logic into the navViewController so
each ionNavView is capable of updating its own child ionView elements.
fix(refresher): Fix refreshing icon in Android 4.3 and lower
feat(clearCache): create $ionicHistory.clearCache()