Allow direct modification of `projectile-command-map'

I wanted to add a second prefix key pointing at the map on C-c p. I
realized that I could duplicate the entire command map by
using (key-binding (kbd "C-c p")) as a define-key target, but if I
wanted to modify the inner map then that binding was order dependent.
Ie, I could modify the inner map by defining it twice, once
for each prefix, or modify one prefix and then add another prefix.

The inner map appears to be defined this way to be more terse, but it
interferes with traditional binding functions. Can we make it more
accessible, or is there some other reason to nest it this way?
Document that `projectile-command-map` is now available
[Fix #348] Add prefix arg support to helm-projectile

With a prefix argument the cache will be invalidated as in `projectile-find-file'.
Merge pull request #327 from dgtized/prefix-map

Allow direct modification of `projectile-command-map'
[Fix #274] Use ag instead of ag-regexp by default in projectile-ag
[Fix #330] Update the current tags table via the find-file-hook
Visit the tags table only if a TAGS file exists
Improve `projectile-project-buffer-p`

Check for hidden/ignored buffers first, then for project root.

Fixes #337
Merge pull request #337 from Silex/master

projectile-kill-buffers tries to reopen old TRAMP buffers
[Fix #326] Rebuild tags-completion-table every time projectile-find-tag
gets invoked
[Fix #78] Add support for sorting displayed files by access and
modification time
[Fix #325] Make it possible to filter a project's buffers
[Fix #167] Add recent-files-first sort order
add gulp to compile commands