Add option to disable day headers completely.

[SimenCodes]

I have the widget configured to only display my agenda for the day, and I found the date headers only take up additional space without really adding any new information. So, I added an option to remove them. This is primarily something I made for myself, but figured other people might be interested too

[yvolk]

Hello @SimenCodes , thank you for the contribution.
I see these problems though that need to be fixed:

1. Please rename in your changes all names/references etc. of "day entries" to "day headers" to avoid confusion.
2. Tests are broken now. Please fix them. (To launch tests locally in "Android Studio" right-click on the app\calendar-widget\src\androidTest\java folder and select "Run All tests" menu item...)

or see Travis build https://travis-ci.org/plusonelabs/calendar-widget/builds/207890973#L3228 , line 3228:

Error in testBirthdayOneDayOnly:
org.json.JSONException: No value for showDayEntries
at org.json.JSONObject.get(JSONObject.java:389)

[SimenCodes]

Hi @yvolk, thanks for the feedback. Have updated all references to "day headers" now, and added the missing JSON value to make the tests run again.

[yvolk]

@SimenCodes Please look how your feature works with the new "Single line layout" #42 (comment)