Fix issue 115: rename "internal" to "associated" headers.
Both terms were used for the same concept (a header with the same base name as its associated source file, e.g. foo.h/foo.cpp) inconsistently within the code base.
No functional change.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@501 812d1f90-f944-d88a-411a-122bdc93e1ea