Support changeHistory nodeset

[clnsmth]

For communicating changes between data package revisions.

Implementation ideas:

1. User calls make_eml() with return.obj = TRUE, constructs and adds the changeHistory nodeset themselves, then calls EML::write_eml(). NOTE: This requires a solution to the workaround at make_eml() line 1382.
2. template_change_history(), a new function returning a tab-delimited template with changeHistory nodes as fields.

[clnsmth]

Much of template_change_history()'s return could be auto-generated based on a sequence of EML files. This may lower the activation energy enough for users to actually report this info.

Related to:
PASTA issue 41
PASTA issue 48