Backport generateId from supercluster.js (#27)

This change backports generateId feature from js library. The feature
allows to generate unique feature ids if Options.generateId is set to true.

Merge pull request #25 from mapbox/zmiao-minpoints-option

Introduce minPoints option

Update to geometry v1.0.0

mark more methods as const

std::round is not defined in some platforms (#12)

Use the C version.

GCC 4.9 support

fix getTile clusters near dateline (#4)

fix xcode shadowed field error

fix benchmark