Fixing periodic BCs for BVPs in chebgui. Closes #1136.

Also introduced a periodic BVP demo.
Merge pull request #1138 from chebfun/fix-periodic-GUI-problems

Fixing periodic BCs for BVPs in chebgui. Closes #1136.
Merge branch 'development' of ssh://github.com/chebfun/chebfun into feature-firstOrder-reform

Conflicts:
	@chebgui/solveGUIbvp.m
Suppressing printing to console when IVP/FVP is detected.
Fixing bug spotted by @trefethen for N.op = @(u) diff(u).