Add default implementation of bit vector.

This implementation is based on an array of long, and does not include 
'rank' and 'select' yet.
Remove redundant "public" annotation from interface methods
Apply changes of review

The changes commented in the code review are already applied.
Include bit vector iterator.
Apply changes of second review
Apply minor changes
Merge pull request #3 from Wikidata/implementation-of-bit-vector

Add default implementation of bit vector.
Merge pull request #6 from Wikidata/datamodel-interfaces-cleanup

Remove redundant "public" annotation from interface methods
Updated datamodel for time

Changes based on discussions in Berlin last week:
* integer seconds
* timezone offset in integer minutes
* tolerance ofr "before" and "after"
New EntityDocuments to replace EntityRecords

This change implements results from the datamodel discussion
in Berlin last week. Future EntityDocuments will be needed
for queries and media pages on Commons. Not all of them will
have terms (label etc.).
Allow arbitrary strings as entity type

This change is based on the discussion in Berlin last week.
It should be possible to add new entity types in the future,
maybe even in some local configuration, and hence the types
need to be extendable. The strings used to identify the known
types are the IRIs of the corresponding classes in the RDF
export.
Use longs instead of doubles to define coordinates (fixed precision)

Positions are now measured in whole nanodegrees. This might still
change, but the datatype (long) and basic setup will remain.
Remove URLValue

URLValues are currently not used as primitive values
in Wikibase. URLs are represented by StringValues instead.
Name changes for clarity and consistency

ALl Value classes now contain "Value". The IriValue interface
was remaned to IriIdentifiedValue to clarify that it is not 
a primitive value for IRIs.
New classes for representing references
Support for Claim objects
Make Statements refer to Claims instead of extending Claims

A subtle change in the data model as discussed in Berlin
last week. Statements are no longer special kinds of Claims.
The new modeling matches the current public documentation but
is different from the current PHP implementation.
Add licence headers
New MonolingualTextValue type
Rename some elements for consistency