Issue #320 - Date Interval Functionality

Adds the WP_Stream_Date_Interval and Carbon classes, and replaces the
date from / date to fields in the Stream list table nav date with the
interval fields.
Fixed readme.md build problem
Use wp_stream_filter_input().

Instead of checking isset on $_GET.
Update predefined intervals filter name.

From stream-predefined-intervals to wp_stream_predefined_date_intervals.
Merge pull request #325 from lukecarbis/issue-320

Issue #320 - Date Interval Functionality