Parse expressions in Liquid::Variable#parse.

cc @camilo, @sunblaze, @grollest

## Problem

Right now we are doing the expression parsing during liquid rendering.  Since we memoize the result, this doesn't cause problems when always doing a parse followed by a render.  However, this reduce the benefits we can get from caching the parsed liquid template to re-use, since it won't include expression parsing.

## Solution

Pull #391 already separated `Context#[]` into `Expression.parse` and `context.evaluate`, so was just a matter of using these two new methods in the Variable class.

## Benchmarks

I used pull #442 to benchmark the results so that it is easier to see how this distributes the time spent parsing and rendering.

Using master code:

```
~/src/liquid(benchmark-load-render)$ bundle exec rake benchmark:run
Rehearsal ----------------------------------------------------------
parse:                   3.770000   0.050000   3.820000 (  3.819407)
marshal load:            1.320000   0.010000   1.330000 (  1.326707)
render:                  4.400000   0.040000   4.440000 (  4.447144)
marshal load & render:   5.780000   0.020000   5.800000 (  5.806654)
parse & render:          8.130000   0.030000   8.160000 (  8.154378)
------------------------------------------------ total: 23.550000sec

                             user     system      total        real
parse:                   3.600000   0.020000   3.620000 (  3.619495)
marshal load:            1.320000   0.010000   1.330000 (  1.331599)
render:                  4.260000   0.020000   4.280000 (  4.279742)
marshal load & render:   5.720000   0.020000   5.740000 (  5.748194)
parse & render:          8.320000   0.050000   8.370000 (  8.368795)
```

using code from this branch:

```
~/src/liquid(completely-parse-variables)$ bundle exec rake benchmark:run
Rehearsal ----------------------------------------------------------
parse:                   5.140000   0.040000   5.180000 (  5.186593)
marshal load:            1.680000   0.010000   1.690000 (  1.687120)
render:                  3.450000   0.030000   3.480000 (  3.482392)
marshal load & render:   5.200000   0.030000   5.230000 (  5.225160)
parse & render:          8.650000   0.040000   8.690000 (  8.690502)
------------------------------------------------ total: 24.270000sec

                             user     system      total        real
parse:                   5.070000   0.030000   5.100000 (  5.102806)
marshal load:            1.560000   0.000000   1.560000 (  1.560547)
render:                  3.340000   0.020000   3.360000 (  3.360897)
marshal load & render:   5.340000   0.030000   5.370000 (  5.367271)
parse & render:          8.810000   0.050000   8.860000 (  8.860455)
```

So these changes increase the time to parse and decrease the time to render as expected.

The reason parse & render takes longer now is that we no longer memoize expressions that appear multiple times in the source.  This could be done, but we don't really have a proper parse context, unlike rending which has a Liquid::Context object to store the cached expressions.
why did '' change to nil here?