small parformance improvements; add unit test for contexts
remove unnecessary static data members from Log class
Added LOG_CONFIG_PROP macro to configure from string data
initialize and configure logging properly; use LSST_LOG_CONFIG envvar
update mainpage.dox and some doxygen comments
disallow empty context names in LOG_PUSHCTX and LOG_CTX, exception will be raised
Merge branch 'u/salnikov/DM-1180'
add do {} while around all ifs in the LOG macros
Merge branch 'u/salnikov/DM-1186'
Change fmt param from std::string to char const*.

This is necessary for standards compliance in va_start.  Compilers are
starting to complain about this.