Allow curly brackets and escaped quotes in Variables (2)

The sequel to https://github.com/Shopify/liquid/pull/170

With this patch this should work now:
```
{{ '{\'message\': \'this should work now\', \'foo\': {\'bar\':\'rab\'}}' }}
```

the main idea was to use it for a i18n filter accepting json encoded strings as argument
```
{{ 'translation_string' | t:'{\'count\':10,\'custom_var\':\'hello world\'}' }}
```

the patch should also fix https://github.com/Shopify/liquid/pull/251

Benchmark result of the original current liquid master:
```
/Users/stefan/.rvm/rubies/ruby-2.1.1/bin/ruby ./performance/benchmark.rb lax
Rehearsal ------------------------------------------------
parse:         2.180000   0.010000   2.190000 (  2.189993)
parse & run:   5.040000   0.010000   5.050000 (  5.046477)
--------------------------------------- total: 7.240000sec

                   user     system      total        real
parse:         2.140000   0.000000   2.140000 (  2.144717)
parse & run:   5.090000   0.010000   5.100000 (  5.097024)
```

Benchmark result after this patch:
```
/Users/stefan/.rvm/rubies/ruby-2.1.1/bin/ruby ./performance/benchmark.rb lax
Rehearsal ------------------------------------------------
parse:         2.420000   0.010000   2.430000 (  2.429807)
parse & run:   5.220000   0.000000   5.220000 (  5.246784)
--------------------------------------- total: 7.650000sec

                   user     system      total        real
parse:         2.330000   0.000000   2.330000 (  2.335282)
parse & run:   5.410000   0.010000   5.420000 (  5.432291)
```