introducing external dependencies

Adding a way to map a file path to an external dependency to extract
the dependencies binding configuration into the given file path :

for example you're having the following include statement reffering
a binding configuration from an external jar in your binding
configuration :

<include path="./anExternalDependency/myDependencyBindings.xml" />

assuming the jar containing the binding is called external.jar
you can declare in your gradle script the external binding
dependency this way :

JIBXBinding {
    ......
    externalDependencyMapping 'external.jar','anExternalDependency'
	.....
}

Change-Id: I79d2b978d2a66105fc7c8d1014a0c2f364f86f21