Add new API chewing_set_logger
Add unit test for logger
Add log level
Add document for chewing_set_logger
Update version due to chewing_set_logger API
Merge branch 'logger'

* logger:
  Update version due to chewing_set_logger API
  Add document for chewing_set_logger
  Add log level
  Add unit test for logger
  Add new API chewing_set_logger
Add LDFLAG -Wl,--no-undefined

The LDFLAG `-Wl,--no-undefined' will cause linker to report any
unresolved symbols when linking libchewing.so, which is good because
libchewing.so shall not contain any resolved symbols at all.

The original `-no-undefined' does not do this job in GNU/Linux, see [1]
for more information.

[1] http://lists.gnu.org/archive/html/libtool/2009-03/msg00026.html