Fix https://github.com/junegunn/fzf.vim/issues/128

Based on the patch by Matt Westcott (@mjwestcott).
But with a more conservative approach:
- Does not use linearly increasing penalties; It is agreed upon that we
  should prefer matching characters at the beginnings of the words, but
  it's not always clear that the relevance is inversely proportional to
  the distance from the beginning.
- The approach here is more conservative in that the bonus is never
  large enough to override the matchlen, so it can be thought of as the
  first implicit tiebreak criterion.
- One may argue the change breaks the contract of --tiebreak, but the
  judgement depends on the definition of "tie".

[bash] Fix vi mode pre-launch delay

Summary:
Fix adapted from [@adamheins: fzf, vi-mode, and fixing delays][1].

  [1]: https://adamheins.com/blog/fzf-vi-mode-and-fixing-delays

The basic problem is that
fzf presses <Esc> to enter vi-movement-mode
(as opposed to insert mode)
and then presses a bunch of keys to set up the buffer.
But the <Esc> keypress is also the prefix for a bunch of other commands,
so Bash will dutifully wait an excruciating half-second
before actually executing this command.
Instead, we bind <C-x><C-a>, which is unused by default
and seems reasonably unlikely to be custom-bound,
to be another way to enter vi-movement-mode;
this binding is unambiguous, so fzf can use it without delay.

This change was made by just `:s/\\e/\\C-x\\C-a/gc`
in the relevant section,
after adding the actual binding and comment at the top.

https://github.com/junegunn/fzf.vim/issues/114

Fix algorithm tests

- Do not create zsh files if zsh is not installed (@adam8157)
- Use command -v instead of which (@netei)
- Reenable --pre option

Close #531

Fix typo in README.md

Close #526

Patch submitted by @netei. Close #524.

Related: https://github.com/junegunn/gv.vim/issues/19

Close #500

This fixes the problem where a new tab page is not closed when the
following configuration is used:

  let g:fzf_layout = { 'window': 'execute (tabpagenr()-1)."tabnew"' }

Close #516

Close #513

Close #511

[vim] s:callback: Always return list.

Fixes "E706: Variable type mismatch for: ret" when an exception is
caught.

Related: #470, #497

- GOPATH is no longer required
- fzf repository does not have to be in GOPATH
- Build Linux binary with Go 1.5.3

- Slightly more efficient processing of Options
- Do not return reference type arguments that are mutated inside the
  function
- Use util.Constrain function when appropriate

Go 1.3 compatibility