examples/uhs*.xml:

Restructured UHS schema examples.
Merge branch 'master' of github.com:gem/oq-nrmllib
schema/hazard/uhs.xsd:

New UHS schema definition.
schema/nrml.xsd:

Added `uniformHazardSpectra` and clean up the ordered allowed top-level
elements.
hazard/writers:

Abstracted BaseCurveXMLWriter, to be extended by HazardCurveXMLWriter and
UHSXMLWriter.
hazard/writers:

Sketched out UHSXMLWriter. The constructor is complete (with a few validation
checks), but the `serialize` method needs to be implemented.
tests/hazard/writers_test:

Added validation tests for UHSXMLWriter and stubbed out 3 serialization tests.
tests/hazard/writers_test:

Added `UHSData` mock object for testing serialization.
hazard/writers:

Corrected copyright date in license header.
tests/hazard/writers_test:

Corrected copyright date in license header.
hazard/writers:

Implemented `serialize` method of UHSXMLWriter.
tests/hazard/writers_test:

Implemented serialization tests for UHSXMLWriter. Includes cases for:
    * logic tree end branch
    * mean
    * quantile
hazard/writers:

Slight correction to the docstring of HazardCurveXMLWriter.
Merge pull request #81 from larsbutler/uhs

UHS schema and XML writer

[r=gvallarelli] [f=1154108]