Enrich the flot with the ability to add text into markings.

Adjust some indentation and use === instead of == recommended by Travis CI.

Add an 'aboveGrid' option to the markings for backwards-compatibility.

Fix the indentation issues.

Use !== instead of != as Travis CI suggests.

Use addText API to get rid of the redundant code; Update API.md.

Update some line ending issues.

Add a newline to the end of the file.