That looks very much like crown vetch.

Crown vetch is a viny weed of the pea family.

I think my Mom helped nail this one. She called it Sensitive Plant, though it may actually be in a different Genus. It looks most like Yellow-puff (Neptunia lutea) from what I can tell.
http://en.wikipedia.org/wiki/Neptunia_lutea

{{gwi:1347161}}

Now the tricky part... how do I kill it without hurting my St Augustine?