Adding a new constructor for InternedString that takes a 64bit signed integer and convert it to string.

The implementation uses a static map from numbers to the InternedStrings for quick instantiation.
Also it results on the same InternedString created if passing the corresponding numeric string.
Using it in SceneCache for quickly finding the sample entry names.
Making sure the g_numbers map is initialized before any static numeric InternedString is instantiated.
Merge pull request #42 from ldmoser/numericInternedString

Adding a new constructor for InternedString that takes a 64bit signed integer and convert it to string.