Update minimum protobuf version.

This compile issue https://code.google.com/p/protobuf/issues/detail?id=121 seems to affect node-mapnik trunk under GCC 4.4.3/linux, with errors like

    /usr/include/google/protobuf/repeated_field.h: In member function ‘void google::protobuf::internal::RepeatedPtrFieldBase::SwapElements(int, int)’:
    /usr/include/google/protobuf/repeated_field.h:630: error: no matching function for call to ‘swap(void*&, void*&)’
Merge pull request #176 from rcoup/patch-1

Update minimum protobuf version.
update composite ops and report image i/o availability correctly per Mapnik 2.3.x
Merge branch 'master' of github.com:mapnik/node-mapnik
for raster datasources also report 'raster' for geometry type
further flesh out mapnik.supports constants
bump to v1.2.1
add webp image view encoding test - refs mapnik/mapnik#2024
consistent tmp directory notation
only run webp test if mapnik has support