first stab at POSIX-style softlinks
fixed is_dangling() test
allow underscores but explicitly look for  prefixes in attribute names (to make  etc. work on target nodes)
__getitem__, __setitem__, __str__, copy, move, remove, rename all tested and working for Array and Table targets. TODO: tab autocompletion from target node's attributes?
test case for get/set target node attributes via softlink
fixed for case where softlink-->group.child.attribute ('Group' object has no attribute '<name>')
added test cases for softlink-->group.child.attribute and softlink-->softlink-->group.child.attribute
Merge branch 'enhanced_softlinks' into develop
Update reease notes (closes: gh-399)
Fix sphinx warnings
Update sphinx conf.py and sphinx build makefiles