Adds a new get_size() method

Implements a new hpack_header_field_get_size() method to the Header
Field class.
Merge pull request #36 from Akrog/issue#35

Fix #35: Libchula's *buf* should be unsigned
Fixes a few compilation warnings
Libchula's test complains with HAVE_STRCASESTR

When cmake finds strcasestr and defines HAVE_STRECASETR then the
compiler complains about `implicit declaration of function
‘strcasestr’` in *libchula/test/missing_sysfuncs_test.c*.

The reason for this is that the function declaration is conditioned to
*__USE_GNU* in *string.h*.

All that's needed to fix this is defining the macro _GNU_SOURCE before
any #includes.