Various enhancements

Hello,

I came across your plugin and found it very interesting. First, thank you for it. It's really useful.

Then as I need to work on multiple projects, a jira server configured with self-signed certificates and so on, I've adapted vim-jira-complete to my needs.

Here is the result : 

- The completion keybinding can be configured in the .vimrc
- The text inserted can be configured (I often need the title of the issues) -- ideally, the format string should have been implemented in python ; it should be easy to fix for someone who knows python well.
- Options are made project aware (so we can work simultaneously of several projects, each one being supervised by a different JIRA)
- Options can be passed to the Python _requests_ library (in order to work around self-signed certificates)
- The code is dispatched to an autoload plugin in order to minimize plugin footprint
- Passwords are not automatically cached.
- Completion will take the characters before the cursor into account to restrict the possibilities
- Issues fetch from the Jira server are cached by default
- An API is available (through autoloaded functions) to fetch jira data from snippets for instance.


In the process, I've introduced a dependency to an API plugin of mine (lh-vim-lib). The two functions used in my patch (lh#option#get(), and `GetCurrentKeyWord()`) can easily be imported to make vim-jira-complete self sufficient. Both functions are under a [GPL with exceptions license](https://code.google.com/p/lh-vim/wiki/License) -- the exception does not really apply in their case.

If you accept this pull request, you'll have to adapt the vim-addon-info file, a few various headers, and the README.

At this time, errors from python library are incorrectly reported. I'll have to look into it someday.

Best regards,