Finish initial development of VBA-XML project.

[AndrewPullon]

Hi Tim,

Long time user of VBA-Web and wanted to give back to the project. Here is my implementation of VBA-XML as inspired by your design.md and the coding style used in the VBA_JSON project. I have integrated this version of VBA-XML into VBA-Web (i.e., into the WebHelpers module) and will make a pull request on that project when I have the time.

Notes on this pull request:

• Inspiration is drawn heavily from VBA-JSON project coding style.
• I note that this project suffers from speed issues when dealing with XML strings larger than 1,000,000 characters (whereas VBA-JSON handles strings of this size without issue). I've not been able to improve on this performance issue (maybe someone else can?).
• To try and mitigate some of the speed issues I made two design decisions:
• 1. Included support for 'DOMDocument' on Windows machines, as it is significantly faster. Included in this decision is a setting 'XmlOptions.ForceVbaXml' which, if set to 'True' will disable the use of 'DOMDocument' on windows machines.
• 2. Included the setting 'XmlOptions.IncludeNodeMapping' that by default excludes the node mapping (i.e. parentNode, firstChild, lastChild) usually included in XMLNodes. These can be re-introduced to the data structure by setting this option to 'True'.
• I've taken the liberty of a design decision to have the 'nodeValue' for each XML node to be converted to the appropriate variable type. Looking at documentation for DOMDocument, 'nodeValue' is usually 'null' or simply the same as the 'text' property. I thought having the text value parsed to a double, boolean, date etc. would be more useful, so did this in the 'xml_ParseValue' method.
• I didn't implement support for comments in the XML string passed to the 'ParseXml' method. This should probably be done at some point.