Updated API
Smarter Makefile; Added buffered_input.h; Began unit testing

1) Makefile only rebuilds when it needs to
2) Added functions for bounds checking
3) Uses buffered_input.h for readline / skipline etc
   - added buffered reading (10X faster for old zlib libraries)
4) Added outline of proper unit testing in strbuf_test.c
Wrote tests; caught some recently introduced bugs; updated buffered_input API

string_buffer.c / strbuf_test.c:
1) Wrote tests for all functions
2) Bug fixes for the following functions:
   * strbuf_chomp (last commit caused premature nul-termination)
   * strbuf_insert (last commit misbehaved in cases of overlapping src, dst)
   * fixed termination behaviour of strbuf_sprintf_noterm
3) added strbuf_skipline_buf strbuf_gzskipline_buf

buffered_input.h:
1) added fputs2, gzputs2
2) Changed buffered_input api -- readline takes 3 args (char*,len,size) instead
   of single output struct