xml-schema (xsd) for VEP on branch master

Hi Ensembl,

find below a  **XML/XSD** schema for the XML output of REST+VEP.

As stated in the file, this document is not normative.

I tested it on a large number of grch37 variants :
```bash
$ xmllint --schema vep.xsd "http://grch37.rest.ensembl.org/vep/human/region/3:38674586-38674587:1/TAG?content-type=text/xml"
```

and it can be used to generate a java parser for VEP with the standard java tool **XJC**:

```bash
$ xjc vep.xsd

parsing a schema...
compiling a schema...
generated/AbstractConsequences.java
generated/Data.java
generated/IntergenicConsequences.java
generated/MotifFeatureConsequences.java
generated/ObjectFactory.java
generated/Opt.java
generated/RegulatoryFeatureConsequences.java
generated/TranscriptConsequence
```