Added xy2ne option to other funcs in mpl
Updated docstrings of mpl funcs with xy2ne

points and square have xy2ne flag. This updated the docstrings.
Entry on changelog for xy2ne in mpl
Merge branch 'master' into vis-xy2ne

Conflicts:
	doc/changelog.rst
Merge pull request #94 from leouieda/vis-xy2ne

Added `xy2ne` option to other functions in `fatiando.vis.mpl`