add user agent header

fixes #628
Merge pull request #628 from mapbox/user-agents

Set user agent to something useful
report version number in user agent string

refs #628
make sure it also works when there are no git tags
don't print more information when a tag actually is available
support fewer parameters
nu_static_libs
add missing gyp file
Merge pull request #705 from mapbox/user-agent-os

Include operating system and actual version number in User Agent string
Tidy up HeadlessView
don't include the source file in every library
also install the version header
Merge pull request #709 from mapbox/version-header-only

Include version symbols only in mbgl-core library
fix typo
Merge pull request #708 from mapbox/headless-tidy

Tidy up HeadlessView
wrap mbgl-core and mbgl-linux in -Wl,--start-group/-Wl,--end-group
Merge pull request #707 from mapbox/start-group

wrap mbgl-core and mbgl-linux in -Wl,--start-group/-Wl,--end-group
Merge branch 'master' of github.com:mapbox/mapbox-gl-native into android-mason

Conflicts:
	platform/default/http_request_baton_curl.cpp
Add Android user-agent info
Merge branch 'android-mason' of github.com:mapbox/mapbox-gl-native into android-mason