Save word freqs in trie instead a dedicated map

freq可以保存在Trie里面，可以节约大约39M内存  (239m vs 200m)。

```sh
# 保存在 Trie里面
Partition of a set of 1400981 objects. Total size = 206311680 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 498458  36 165082352  80 165082352  80 dict (no owner)
     1 512758  37 28714624  14 193796976  94 unicode
     2 357899  26  8589576   4 202386552  98 float
     3  14657   1  1272760   1 203659312  99 str
     4   7412   1   602880   0 204262192  99 tuple
     5     97   0   299032   0 204561224  99 dict of module
     6    256   0   274048   0 204835272  99 dict of type
     7   2054   0   262912   0 205098184  99 types.CodeType
     8   1994   0   239280   0 205337464 100 function
     9    256   0   227800   0 205565264 100 type
<117 more rows. Type e.g. '_.more' to view.>


# 保存在dict里面
Partition of a set of 1749883 objects. Total size = 239172392 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 498459  28 177665544  74 177665544  74 dict (no owner)
     1 861769  49 49002952  20 226668496  95 unicode
     2 357897  20  8589528   4 235258024  98 float
     3  14615   1  1269856   1 236527880  99 str
     4   7375   0   600144   0 237128024  99 tuple
     5     97   0   299032   0 237427056  99 dict of module
     6    255   0   273000   0 237700056  99 dict of type
     7   2043   0   261504   0 237961560  99 types.CodeType
     8   1985   0   238200   0 238199760 100 function
     9    255   0   226896   0 238426656 100 type
<115 more rows. Type e.g. '_.more' to view.>

```

但由于Trie是python实现的，相比于C实现的dict，在lookup时，性能会低一些。  1.126s vs 1.555s

```python
    import time
    start = time.time()
    for a in xrange(10000):
        for w in jieba.cut(u'你认识那个和主席握手的的哥吗？他开一辆黑色的士。'):
            pass

    print time.time() - start

```