enable spellcheck on markdown files

added an autocommand that performs spellcheck on markdown files
Merge pull request #137 from morenoh149/patch-1

enable spellcheck on markdown files
install tmux version >= 1.8

the copy functions for tmux on mac osx depend on the `copy-pipe` function to be available. That function was implemented in tmux version 1.8. This code ensures that tmux version >= 1.8 is installed. The previous code only checked if any tmux version was installed.
made tmux 1.8 check more robust

Now using `brew list tmux --versions`
```
       ls, list [--unbrewed] [--versions] [--pinned] [formulae]
              Without any arguments, list all installed formulae.

              If formulae are given, list the installed files for formulae. Combined with --verbose, recursively list the contents of all sub-
              directories in each formula's keg.

              If --versions is passed, show the version number for installed formulae, or only the specified formulae if formulae are given.

```
returns `tmux 1.9a` on my machine. Grab second part of this string. Creates a Gem::Version object and compares it that way.
Merge pull request #144 from morenoh149/patch-3

install tmux version >= 1.8
Generalize the tmux version logic.

This code should never have been where it was. I was lazy in reviewing #144 which did not appropriately scope the version check to tmux. This fixes #148.
Use the `-g` option for tmux 1.9 compatibility.
Replace ack.vim with ag.vim.

Fixes #141.
No need to configure ack.vim.
Update the README to reflect ack.vim -> ag.vim.
Fix brew install URL

Fixes #150
Merge pull request #151 from ram-nadella/patch-1

Fix brew install URL
Add tmux.conf.local
Update phinze/cask to caskroom/cask

It triggered this warning if brew cask had already been upgraded
to caskroom/cask and we ran rake:

> Warning: Could not tap phinze/homebrew-cask/brew-cask over
> caskroom/homebrew-cask/brew-cask
Merge pull request #156 from slam/tmux_local

Add tmux.conf.local
Don't recursively map all the time

If users remap keys, the behaviour of the things set up in vimrc may change.

An example of this is the common remapping to switch `;` and `:`, which will
cause all of the keyboard shortcuts involving `:` to fail. Switching the *map
statements to their *noremap counterparts fixes this problem, by using the
original meanings.
Merge pull request #158 from ElliottH/mapping

Don't recursively map all the time
Don't copy the contents of an overwritten selection.
Document select-paste vim config.
Clarify the visual selection overwrite docs.