Fix `hp:do-project-grep' function to work.

    * Fix `hp:do-project-grep' function because it raises an error.
    * Add `hp:helm-c-project-grep-source' variable.
    * Add `hp:helm-c-project-grep-init' function.
    * Add `hp:helm-c-project-grep-source' function.
    * Add `hp:preprocess-grep-command' function.
Modify `hp:helm-c-project-grep-init' function to run grep command asynchronously.

    * Asynchronous grep depends on `deferred.el', so you need to install
    `deferred.el'. When you don't install 'deferred.el', `hp:helm-c-project-grep-init'
    function automatically runs grep command synchronously.
    * Add `hp:has-deferred-p' variable and it's used for deciding if
    `deferred.el' is installed or not.
Merge pull request #1 from Shinnya/fix_helm_project_grep

Fix helm-project-grep function