Optimize Post#{next,previous}

Use object equality for comparisons rather than `Comparable#==`, which in turn uses `Post#<=>`, which is slow.

Specifically, the bottleneck call stack is:

```
Jekyll::Post#<=>
Comparable#==
Array#index
Jekyll::Post#{next,previous}
```

Using the block form of `Array#index` and object equality (`equal?`) yielded a 3x performance improvement for `jekyll build` on a large site (1m6.467s -> 0m19.532s).

This assumes that `#next` and `#previous` are only called on Post objects that are actually in `site.posts`. If that's not a safe assumption, I can come up with an alternative fix. However, the one that seemed obvious to me -- defining `Post#==` in terms of `Post#id` -- turned out to actually be slower.

### Details

I did some profiling using ruby-prof and stackprof. Both indicated that Post#<=> was an extreme bottleneck.

Excerpts from ruby-prof graph:

```
--------------------------------------------------------------------------------
                      0.000      0.000      0.000      0.000      59/91042435      Gem::StubSpecification#activated?
                    684.333    141.914      0.000    542.41991042376/91042435      Array#index
  84.49%  17.52%    684.333    141.914      0.000    542.419         91042435      Comparable#==
                    542.419    308.751      0.000    233.66891042376/91113856      Jekyll::Post#<=>
--------------------------------------------------------------------------------
                      0.005      0.003      0.000      0.002     773/91113856      Array#sort
                      0.427      0.247      0.000      0.180   70707/91113856      Array#sort!
                    542.419    308.751      0.000    233.66891042376/91113856      Comparable#==
  67.02%  38.15%    542.851    309.000      0.000    233.851         91113856      Jekyll::Post#<=>
                    180.840    115.968      0.000     64.87291113856/91113856      Time#<=>
                     52.907     52.907      0.000      0.00091113856/91310726      Fixnum#==
                      0.104      0.104      0.000      0.000    138743/138750      String#<=>
--------------------------------------------------------------------------------
...
--------------------------------------------------------------------------------
                    373.077      0.575      0.000    372.502      85720/85720      Array#map
  46.06%   0.07%    373.077      0.575      0.000    372.502            85720      Jekyll::Post#next
                    372.120     29.587      0.000    342.534     85720/171455      Array#index
                      0.082      0.082      0.000      0.000      85720/87096      Fixnum#<
                      0.080      0.080      0.000      0.000     85716/374666      Array#[]
                      0.079      0.079      0.000      0.000     85720/413908      Fixnum#-
                      0.071      0.071      0.000      0.000     85716/616276      Fixnum#+
                      0.070      0.070      0.000      0.000     85720/255886      Array#length
--------------------------------------------------------------------------------
                    371.819      0.464      0.000    371.354      85720/85720      Array#map
  45.90%   0.06%    371.819      0.464      0.000    371.354            85720      Jekyll::Post#previous
                    371.098     29.299      0.000    341.799     85720/171455      Array#index
                      0.099      0.099      0.000      0.000     85720/167276      Fixnum#>
                      0.079      0.079      0.000      0.000     85717/413908      Fixnum#-
                      0.078      0.078      0.000      0.000     85717/374666      Array#[]
--------------------------------------------------------------------------------
```

Excerpts from stackprof analysis:

```
~/Development/mapbox.github.com $ stackprof stackprof-cpu.dump --text --limit 4
==================================
  Mode: cpu(1000)
  Samples: 17557 (0.14% miss rate)
  GC: 1221 (6.95%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      9753  (55.6%)        9753  (55.6%)     Jekyll::Post#<=>
      5760  (32.8%)         946   (5.4%)     Jekyll::Post#previous
      5866  (33.4%)         928   (5.3%)     Jekyll::Post#next
       709   (4.0%)         550   (3.1%)     Hash#deep_merge
~/Development/mapbox.github.com $ stackprof stackprof-cpu.dump --method 'Jekyll::Post#<=>'
Jekyll::Post#<=> (/Users/john/.gem/ruby/2.1.0/gems/jekyll-1.4.3/lib/jekyll/post.rb:144)
  samples:  9753 self (55.6%)  /   9753 total (55.6%)
  callers:
    4938  (   50.6%)  Jekyll::Post#next
    4814  (   49.4%)  Jekyll::Post#previous
       1  (    0.0%)  block (2 levels) in Jekyll::Site#render
  code:
                                  |   144  |     def <=>(other)
 9016   (51.4%) /  9016  (51.4%)  |   145  |       cmp = self.date <=> other.date
  522    (3.0%) /   522   (3.0%)  |   146  |       if 0 == cmp
                                  |   147  |        cmp = self.slug <=> other.slug
                                  |   148  |       end
  215    (1.2%) /   215   (1.2%)  |   149  |       return cmp
                                  |   150  |     end
```

Timing data, before and after:

```
~/Development/mapbox.github.com $ time jekyll build
Configuration file: /Users/john/Development/mapbox.github.com/_config.yml
            Source: /Users/john/Development/mapbox.github.com
       Destination: /Users/john/Development/mapbox.github.com/_site
      Generating... done.

real	1m6.467s
user	1m5.889s
sys	0m0.446s
~/Development/mapbox.github.com $ time jekyll build
Configuration file: /Users/john/Development/mapbox.github.com/_config.yml
            Source: /Users/john/Development/mapbox.github.com
       Destination: /Users/john/Development/mapbox.github.com/_site
      Generating... done.

real	0m19.532s
user	0m19.095s
sys	0m0.393s
```