Accept backtrace from standard input.

Write tests for more functions:
- btp_thread_dup
- btp_thread_cmp
- btp_thread_add_sibling
- btp_thread_get_frame_count
- btp_thread_quality_counts
- btp_thread_quality
- btp_backtrace_dup
- btp_backtrace_get_thread_count
- btp_backtrace_quality_simple
- btp_backtrace_to_text
- btp_backtrace_get_crash_frame
- btp_backtrace_get_duplication_hash
- btp_backtrace_parse
- btp_backtrace_parse_header

Start writing a manual.

Investigate more duplicates to find more normalization procedures.

Write a similarity function, which returns a number 0..1 telling how
much are two backtraces similar.

Write Python bindings.