Proposal: full XPath 1.0 support

Nobody has claimed this yet.

* all XPath 1.0 **axes** that would allow us to e.g. refer to sibling nodes in various ways (see e.g. #85)
* all XPath 1.0 **functions**, including (maybe more): starts-with(), ends-with(), contains(), substring-after(), substring-before(), translate(), normalize-space(), floor(), ceiling(), last(). The first ones are particularly interesting for querying (external) data using predicates
* all XPath 1.0 **paths** (including e.g. /path/to/@attribute and /path/to/@ns:attribute). Not sure how to argue for this yet, but I'm starting to find this very useful for custom client forks.

We've already started incorporating XPath 2 and XPath 3 features, without fully supporting XPath 1.0. It think full support for XPath 1.0 would provide a great basis for further extensions, in particular custom XPath functions as they can then be implemented by simply translating them to pure (complex) XPath 1.0 expressions. One example is extending the [pulldata() function for XML data](https://github.com/kobotoolbox/enketo-express/issues/250#issuecomment-119689871).

Start by reviewing the XPath 1.0 axes, functions, and paths listed in the proposal, along with related issue #85 and the linked pulldata() discussion. Define the supported scope and completion criteria for full XPath 1.0 coverage, including attribute and namespace paths, before implementation.

Written by the indexing model from the issue text.