Fixed bug in esummary parsing that failed on nested data

I mispelled "Structure" in the call to switch() in parse_node(), which led to
structure nodes falling through to the default "xmlValue" function and being
flattened. The fall-through needs to exist, so all node types can be parsed, so
will write a test for each of the node-specific functions.
updated unix permission after pulling from gh
deleted un-needed functions from entrez_summary.r
New test for parsing xml 'structure' nodes
moved tests to /tests/testthat
Merge remote-tracking branch 'origin/master'