Add recursive loading support

If `g:unite_gist_recursive_loading` is 1, it use
`gist#list_recursively` instead of `gist#list`.

See: https://github.com/mattn/gist-vim/pull/158
Add try-catch to examine the presence of a func.

Add try-cache to examine the presence of a new implemented function of
'gist-vim'.
It is required when that user does not use a latest gist-vim in them
environment and (somehow) accidentally specified
'g:unite_gist_recursively_loading' in them vimrc.

If the user specified the 'g:unite_gist_recursively_loading' and
the 'gist#list_recursively' is not found (determined from E117 exception), the
function displays the warnings and use 'gist#list' instead.

See PR #2 conversation for detail (in Japanese).
Merge pull request #2 from lambdalisue/support_recursive_loading

Add recursive loading support