Add GetSchemaIndex, IndexEntry, Index and SchemaIndex

- Add GetSchemaIndex, IndexEntry, Index and SchemaIndex
- Add 'deep-hash' in development and test Gemfile groups
- Remove Request
- Move PATH_ROOT to Constants
- Remove 'DigitalMeasures::' from class references contained in the
  'DigitalMeasures' namespace
Merge pull request #3 from ucirvine/add-get-schema-index

Add GetSchemaIndex, IndexEntry, Index and SchemaIndex