Added polling APIs. Get a single section. Null check
in CourseAPI.

Added all the apis for polling functionality. Also added a way to
get a single section for a course. And there was a null check that
I added to a function in CourseAPI so it wouldn't crash if the course
array passed in was null.
Merge remote-tracking branch 'upstream/master'

Fix SectionAPI conflict.

Conflicts:
	src/main/java/com/instructure/canvasapi/api/SectionAPI.java
Added created_at to Parcelable in Poll.java
Merge pull request #25 from bradylarson/master

Added polling APIs. Get a single section. Null check
Upgrade to Retrofit 1.5.1

No breaking changes found.
Merge pull request #26 from nbutton23/update-retrofit1.5.1

Upgrade to Retrofit 1.5.1