Initialize the framework of latex document.

.gitignore
now ignores files generated by latex and bounding box

Makefile
add basic latex document generation rule
just type `make' to generate `libicpc.pdf'

libicpc.tex
add two-column landscape latex document template using 'jsarticle.cls'
add sample latex code and source code

libicpc.tex
add a line including `section/sample.tex'

section/sample.tex
contents of the "sample section" are written

src/sample/hello_world.cpp
the "hello world" source

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when you put a section,
first you write "\input{section/hoge.tex}" in `libicpc.tex',
second you edit `section/hoge.tex',
then you put source codes into `src/hoge/' and figures into `imgs/hoge'