Care base_dir

Hello,

I encountered two kinds of error when using gitpusher:

* When the directory specified as `:base_dir` in config file doesn't exist
* When relative path is specified as `:base_dir`

Although it's easy to fix them by creating directory and specifying by absolute path,
it's better if I need not care such tiny thing.

So, I wrote this patch. Could you merge it?
If you could, please do `bunde update` because I added RSpec as dependent gem
and then you can run `bundle exec rake spec` to see whether my patch is ok.

Thanks.