Make system dictionary singleton

[mocobeta]

Closes #104

In Janome 4.0.1, every Tokenizer object independently opens the binary dictionary files with mmap=True (default); this can cause 'Too many open files' errors when multiple Tokenizer objects are created.
This would be resolved by making MMapSystemDictionary singleton and reusing them across Tokenizers.

Reproducible test code

# openfiles_mmap.py
import psutil
from janome.tokenizer import Tokenizer

tokenizers = []
for i in range(10):
    tokenizers.append(Tokenizer(mmap=True))
print('Created Tokenizers = %d' % len(tokenizers))

p = psutil.Process()
open_dic_files = list(filter(lambda x: x.path.find('janome/sysdic') >= 0, p.open_files()))

print('Opened dictionary files = %d' % len(open_dic_files))

With janome 4.0.1:

$ python openfiles_mmap.py 
Created Tokenizers = 10
Opened dictionary files = 400

With this PR:

$ python openfiles_mmap.py 
Created Tokenizers = 10
Opened dictionary files = 40

[coveralls]

Coverage decreased (-0.3%) to 86.878% when pulling 0dd31dc on feat/make-mmapdictionary-singleton into 1b25f94 on master.