Allow users to choose between 0 and 1 based lists for prompts.

I added a variable `g:EclimPromptListStartValue` to let users choose between 0 and 1 based lists when using `eclim#util#PromptList`.

When running `eclim#util#PromptList("prompt", ['a', 'b'])` the user would be shown the following if `g:EclimPromptListStartValue = 0`

    0) a
    1) b

    prompt:

And this when `g:EclimPromptListStartValue = 1`

    1) a
    2) b

    prompt: