Introduce newFromNumericId as intermediate step to replace legacy ids

This was suggested by Daniel. Currently we do have many places in the
code where a numeric id is converted to an EntityId object in some
way. This method is supposed to be an intermediate step to get rid
of all numeric ids. If it's used (I have to do this in other patches
since it's used in other modules) it will make it a lot easier to
find and fix code that still relies on numeric ids.
Update EntityId.php
Update EntityId.php
Update README.md
Improve doc
Disable original constructor whem mocking Term
Merge pull request #58 from wmde/term-mock

Disable original constructor when mocking Term
Add convenience methods to Fingerprint

This covers most of the wishes I had in my email.

One thing I did not implemented are hasLabel, hasDescription and
hasAliasGroup methods, similar to the remove... methods.
Merge pull request #30 from wmde/from_numeric_id

Updated inline documentation of EntityId::newFromPrefixedId
Language magic removed and tests added
Rename getAliases to getAliasGroups
Moved setAliases to setAliasGroups rename to other pull request
Merge pull request #59 from wmde/fingerprint-convenience

Add convenience methods to Fingerprint
Update WikibaseDataModel.php
Merge pull request #61 from wmde/getAliasGroups

Rename getAliases to getAliasGroups
Update RELEASE-NOTES.md
Rename getAliases to getAliasGroups
Merge in master
[WIP] Legacy code removal

This commit tackles the initial part of two distinct though related tasks:

* Removal of the old serialization and deserialization code.
These are the toArray and newFromArray methods.

* Swicthing Entity, Item and Property over to contain value objects
rather than the internal serialization format.