Issue #346 Fixed bug where MkDeployKeys XML nodes were not being written

There were a few things that were incorrect. Fixes done are:

1. Adding the <deploykeys/> node if not present (`addIf` directive in constructor).
2. Fixing the XPath strings to look for the `coords` attribute.
3. Converted XML node names and XPath strings to all-lowercase (`deployKeys` to deploykeys`). 

As for 3. above, Xembly converts them to lowercase when generating directives. This means I can create a node by using `add("deployKeys") and the resulting XML will be `<deploykeys/>`. However, I can't retrieve the same node when I reference `deployKeys` in XPath (e.g. when you call `xml().nodes("/deployKeys")`. This seems like a bug either in JCabi-XML or Xembly; I think it's the latter because as far as I know XML node names are case sensitive.