Version 5.2 release candidate -rc3

[sfeam <sf...@us...>]

Source tarball for -rc3 in now in the "Release Candidates" folder on SourceForge.

I expect this will be the final release candidate.  
If nothing unexpected comes up we can put out version 5.2 itself later in August.

CHANGES SINCE rc2
=================
* NEW: revised "plot with table" can handle string-valued output columns
* CHANGE: revised pipe support on Windows when built with MinGW-64 or MSVC
* FIX: minor bugs in use of gnuplot variables by "fit" command

	Ethan