Initialize log4cplus early if possible.

Use GCC constructor function attribute when it is present.
Check for init_priority GCC variable attribute and use it.
Fix log level string constants initialization issue. (SF bug #303)
Based on patch by: Ray Logel
Bump log4cplus library's libtool revision number.
AUTHORS: Mention Ray Logel.
Remove non-C++11 atomics configuration chekcs.
.travis.yml: Try to fix Travis CI builds.
loglevel.cxx: Insert to/from string/log level conversion functions to the
beginning of vector to favour user defined log levels and conversion functions.
Merge pull request #37 from wilx/master

Merge.