Add AttributeInteger for writing int values

[systemed]

Currently we offer only AttributeNumeric for writing numeric values from Lua. This is mapped to a (4-byte) float in the vector tile. Floats are lossy and potentially less efficient.

This adds an AttributeInteger method, which writes int values to the vector tile. Internally this is currently stored as a double (so much less lossy), and on output to the vector tile it should use fewer bytes in most circumstances.