bugfix: test/test_hashtable segment fault

In zc_hashtable.c, zc_hashtable_remove() and zc_hashtable_del() didn't check the
argument pass to it.  So, if pass NULL to them, they will dereference it, and if
we are lucky, it will segment fault.

This commit fix that problem.
.gitignore: dot files and TAGS file

Emacs stuff:

    o dot files: dir-locals.el
    o TAGS: generated by etags
makefile: add TAGS target
Merge pull request #40 from mofaph/misc

make: 细微的修改（makefile 和 .gitignore）
Merge pull request #39 from mofaph/bugfix-38

尝试修复 #38 出现的段错误