Lock mutexes with `std::lock_guard`
Make the tile directory structure conform to the TMS spec

Previously all tiles were dumped into a single directory.
Updated version number to v0.3.0

* Rebrand the project from 'terrain' to 'ctb'
* Rename the `terrain-build` tool to `ctb-tile`
* The tile output now conforms to the TMS directory spec
* Tile generation using `ctb-tile` is now multithreaded
* Customise the feedback provided by `ctb-tile`
* Various fixes