Simple expressions

Added (I think) enough support for basic arithmetic and comparison operators to do necessary formatting and filtering just by adding built-in functions to the export language.

Example query that now works (this is the JSON form of it that will get stored into couch or output by some parser)

```
{"Map": {
    "source": {"Ref": "xform_es[*]"},
    "body": {"Apply": {
        "fn":{"Ref": "len"},
        "args":[{"Ref": "form.log_subreport.log[*]"}]}}}}
```

Output on my demo domain: `[6, 6, 0, 0, 0, 0, 0, 0, 0, 0]`