Fix a bug causing element having text node child to be considered as leaf node, and add extraction for value property

The bug behaviour, for entity like this one:
```
<us-gaap:AccountsPayableCurrent contextRef="I2012Q4" decimals="-6" id="Fact-84774E2BABC3C1BE0251D7A597ED2610" unitRef="usd">65000000</us-gaap:AccountsPayableCurrent>
```
xCurator “attributize” method converts the attributes to child elements:
```
<us-gaap:AccountsPayableCurrent contextRef="I2012Q4" decimals="-6"
    id="Fact-84774E2BABC3C1BE0251D7A597ED2610" unitRef="usd”>65000000
    <contextRef>I2012Q4</contextRef>
    <decimals>-6</decimals>
    <id>Fact-84774E2BABC3C1BE0251D7A597ED2610</id>
    <unitRef>usd</unitRef>
</us-gaap:AccountsPayableCurrent>
```
Now the BasicSchemaExtraction step still considered this new element as a leaf node, and stopped further extracting its underlying schema.

I made the following changes:

1. changed the ifLeaf implementation so that only element having no child element is leaf node
2. added discovery for value property from text node in basic schema extraction

So now the generated schema will contain the value property extracted from the text node of non-leaf element (if the text node exists).

```
<entity path="/xbrli:xbrl/us-gaap:AccountsPayableCurrent" type="http://www.sec.gov#us-gaap:AccountsPayableCurrent">
    <id>http://www.sec.gov#${UUID}</id>
    <property key="false" name="http://www.sec.gov#id_property" path="id/text()"/>
    <property key="false" name="http://www.sec.gov#decimals_property" path="decimals/text()"/>
    <property key="false" name="http://www.sec.gov#value_property" path="text()"/>
    <property key="false" name="http://www.sec.gov#contextRef_property" path="contextRef/text()"/>
    <property key="false" name="http://www.sec.gov#unitRef_property" path="unitRef/text()"/>
</entity>
```