Allow metadata to be retrieved from source containing data for multiple entities

[reggieb]

Sometimes the source of SAML Metadata publishes a particular set of configuration data within a larger set of data for many SAML entities. The issue has been raised previously in #346

Where this is the case, <md:EntityDescriptor> may not be defined at the root of the Metadata XML, but is within <md:EntitiesDescriptor>.

This modification allows for the three use cases I have seen:

1. <md:EntityDescriptor> is at the Metadata root and the data refers to only one SAML entity (currently handled)
2. <md:EntityDescriptor> is within <md:EntitiesDescriptor> and the data refers to only one SAML entity
3. <md:EntityDescriptor> is within <md:EntitiesDescriptor> and the data refers to many SAML entities

For case three, the particular entity descriptor data to be retrieved can be specified in the parse call (as detailed in the updated README)

[reggieb]

One of the Travis builds has stalled:

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

Is that a JRuby Nokogiri issue?

[pitbulk]

Hi @reggieb

sorry for the delay.

I included your PR in a another PR-389:

[reggieb]

OK. thank you.