Logging Improvements

[gegles]

• Replaced log4cxx by spdlog
• Header-only -> one less dependency to compile cross-platform
• Create super think logging facility to stub-out or replace spdlog
• Make logging framework optional

• Minor cleanups

  Some things still need to be improved: log config, logger names, etc..

[gegles]

Will do. Thanks @gabime! That's quite the "customer" service (or should I say community service)!

[gabime]

@gegles :+1 Those macros translate to empty statements in spdlog by default. To turn them on in debug builds you'd need to define 'SPDLOG_TRACE_ON' and 'SPDLOG_DEBUG_ON' before including spdlog.h