Change parsing to look for blank line delineating captions, rather than looking for the integer beginning the next line.

This fixes an issue where the text of the caption is composed of a single integer, which throws off the old way of parsing.
Add test for parsing SRT file with captions that only contain numerals.
Add tests data for numeric SRT parsing
Remove a newline in the sample DFXP data that caused several conversion tests to fail.
Merge pull request #14 from jaylyerly/master

Parse SRT based on blank lines instead of integers.