Allow schema to pass in hex separator value.

By doing this we can pass in any field separator instead of using a restricted set or having to extend the class.
So say I need to generate a csv file, I can have each line details configured as 
``` xml
<Schema>
	<field id="prod-description" type="A" separator="2C" length="50" /><!--0x2c is a comma-->
	<field id="currency-code" type="A"  separator="2C"  length="20"/>
	<field id="upc" type="A"  separator="2C"  length="20" />
	<field id="request-type" type="A" separator="DS"  length="20" />
</Schema>
```
This will output 
prodDescription,currencyCode,upc,itc.

Packing and unpacking seems to work in my local tests.