[Project] Examples has no tests, actually.
[Bug Fix] Fixing build on OS X (closes #4)

By enforcing google testing framework to use own tuple implementation, instead of <tr1/tuple>, which is legacy.
Also rearranged some function calls in CMakeLists.txt.
[Elasticsearch] Using logging macro without severity level.
[Elasticsearch] Introduced sink config.
[Elasticsearch] Elasticsearch client now strictly starts after sink's worker thread started.
[Elasticsearch] Tiny reorganizing. Added concepts for pool and balancing.
[Elasticsearch] Further decomposition.
[Elasticsearch] Some tasks added.
[Elasticsearch] Added connection skeleton.
[Elasticsearch] Fixed warnings.
[Elasticsearch] Pool is now meant to be thread safe (but it isn't at this moment).
[Elasticsearch] Pool now returns copy of shared pointer, instead of reference, which is unsafe.
Add scoped attributes.

scoped_attributes_t object automatically adds some attributes to all log messages in its scope.
Added abstract base class for scoped attributes.

User may want to implement his own guard.