Search for expression if element is not found in ggb reader

This fixes part of https://github.com/jsxgraph/jsxgraph/issues/99
The relevant xml:
``` xml
<expression label="C" exp="A + b" type="point" />
<element type="point" label="C">
	<show object="true" label="true"/>
	<objColor r="0" g="0" b="255" alpha="0.0"/>
	<layer val="0"/>
	<labelMode val="0"/>
	<coords x="5.0" y="3.0" z="1.0"/>
	<pointSize val="3"/>
	<pointStyle val="0"/>
</element>
<command name="Vector">
	<input a0="A" a1="A + b"/>
	<output a0="v"/>
</command>
```
When the reader parses the Vector v command, it searches for element with a label "A + b".
This PR also searches for an element which has an expression "A + b".