conditional logging

Add create_async factory functions for async loggers

Fix for MinGW error: 'There are no arguments that depend on a template parameter'

Signed-off-by: Remigiusz Kołłątaj <remigiusz.kollataj@gmail.com>

UTC Time

use if statement instead of switch (changes of adding new time specifications outside of standard are probably unlikely anyhow)
pattern_time -> pattern_time_type
ptime variable name -> pattern_time variable name
make sure four spaces used, not tabs

_pattern was never set... but it was also never used. Intentional by @gabime, or a reflection of a refactor with some data member left behind?

implement a flag (in this case, an enumeration) that allows control over the type; we make it an enum for possible expansions of time abstractions that might make it into the C++ standard in the future (see Howard Hinnant's date/timezone library) or might be usefully-available from the OS at some point in time

Fixed issues 449

Please check it