Add barebones projectile.vim support

References #15.
Prefer projectile.vim's :A

References #15.
Require g:rake_legacy to use old :R commands
Use projectile for navigation commands if available

Closes #15.
Document projectile.vim dependency
Restore :Elib default of gemspec
Provide default dispatch and compiler
Use projectile's "compiler" replacement "make"
Warn even when g:rake_legacy is enabled
Favor {file} over %

Projectile is going to support using the alternate to determine
dispatch.
Binstubs in projections
Tentatively drop ctags

This should be generalized and moved into either projectile.vim or some
other plugin, but I've been using Git hooks for ctags for years now and
just don't care to bother.  Let's see if anyone even notices it vanish.
Fix navigation command explanation
Let projectile.vim auto-chdir for rake
Alternate strategy for :Etest/:Espec precedence
Hedge bets on projectile.vim rename
Allow projectile to handle transition
Improve test executable logic
Switch to projectionist
Add default dispatch for Rakefile and gemspec