Separated the Locator Service from the Locator itself.

This makes the codebase a bit more clean, as well as allows clients to
resolve Locator Service like a normal service.
Locator cleanup.
Proper endpoint creation code style.
Fix selecting of a service from a routing group.

Locator must not inspect groups for which sum of weights equals zero (not "doesn't equal"), because such groups don't have an active services.
Merge pull request #63 from andrusha97/routing-groups-fix

Fixed selecting of a service from a routing group.
Merge branch 'master' of github.com:cocaine/cocaine-core
Do not add Locator to the Router.

According to @andrusha97 comment for commit
f4655b4e9fe3af8b72c58e30624b2d9ee16cd35b, Location Service shouldn't be
added to the Router.
Synchronizing branch 'master' of https://github.com/cocaine/cocaine-core into StructuredLogs

Conflicts:
	include/cocaine/context.hpp
	src/context.cpp