Fix issue #93. Restore default mappings for GCC
- iwyu.gcc.imp is no longer added automatically if no mapping file is specified
- Instead, the defaults are hard-coded in iwyu_include_picker.cc
- Fix some 80-column overruns

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@445 812d1f90-f944-d88a-411a-122bdc93e1ea