Use %z instead of %Z to format date in article

When use %Z for date formatting, it does not always the same time when parsing that again. That is why it complain 'filename doesn't match the date in its frontmatter', see #143

```
>> Time.zone = 'Beijing'
=> "Beijing"
>> t = Time.zone.now
=> Wed, 23 Apr 2014 14:08:22 CST +08:00
>> t.strftime('%F %R %Z')
=> "2014-04-23 14:08 CST"
>> Time.zone.parse "2014-04-23 14:08 CST"
=> Thu, 24 Apr 2014 04:08:00 CST +08:00
>>
```

We can list all the TimeZone abbreviations below, there are some abbreviations conflicts in different time zones. It shouldn't be used to parse from.

## Time zone abbreviation conflicts

```
CST (GMT+08:00) Beijing",
CST (GMT+08:00) Chongqing",
CST (GMT+08:00) Taipei",
CST (GMT+08:00) Urumqi",
CST (GMT+09:30) Adelaide",
CST (GMT+09:30) Darwin",
CST (GMT-06:00) Central America",
CST (GMT-06:00) Saskatchewan",

WST (GMT+08:00) Perth",
WST (GMT+13:00) Samoa",
```

## To get all time zone abbreviation list

```
ActiveSupport::TimeZone.all.map{|tz| Time.now.in_time_zone(tz).strftime("%Z") + " " + tz.to_s}
[
    [  0] "SST (GMT-11:00) American Samoa",
    [  1] "SST (GMT-11:00) International Date Line West",
    [  2] "SST (GMT-11:00) Midway Island",
    [  3] "HST (GMT-10:00) Hawaii",
    [  4] "AKDT (GMT-09:00) Alaska",
    [  5] "PDT (GMT-08:00) Pacific Time (US & Canada)",
    [  6] "PDT (GMT-08:00) Tijuana",
    [  7] "MST (GMT-07:00) Arizona",
    [  8] "MDT (GMT-07:00) Chihuahua",
    [  9] "MDT (GMT-07:00) Mazatlan",
    [ 10] "MDT (GMT-07:00) Mountain Time (US & Canada)",
    [ 11] "CST (GMT-06:00) Central America",
    [ 12] "CDT (GMT-06:00) Central Time (US & Canada)",
    [ 13] "CDT (GMT-06:00) Guadalajara",
    [ 14] "CDT (GMT-06:00) Mexico City",
    [ 15] "CDT (GMT-06:00) Monterrey",
    [ 16] "CST (GMT-06:00) Saskatchewan",
    [ 17] "COT (GMT-05:00) Bogota",
    [ 18] "EDT (GMT-05:00) Eastern Time (US & Canada)",
    [ 19] "EDT (GMT-05:00) Indiana (East)",
    [ 20] "PET (GMT-05:00) Lima",
    [ 21] "PET (GMT-05:00) Quito",
    [ 22] "VET (GMT-04:30) Caracas",
    [ 23] "ADT (GMT-04:00) Atlantic Time (Canada)",
    [ 24] "GYT (GMT-04:00) Georgetown",
    [ 25] "BOT (GMT-04:00) La Paz",
    [ 26] "CLST (GMT-04:00) Santiago",
    [ 27] "NDT (GMT-03:30) Newfoundland",
    [ 28] "BRT (GMT-03:00) Brasilia",
    [ 29] "ART (GMT-03:00) Buenos Aires",
    [ 30] "WGST (GMT-03:00) Greenland",
    [ 31] "UYT (GMT-03:00) Montevideo",
    [ 32] "GST (GMT-02:00) Mid-Atlantic",
    [ 33] "AZOST (GMT-01:00) Azores",
    [ 34] "CVT (GMT-01:00) Cape Verde Is.",
    [ 35] "WEST (GMT+00:00) Casablanca",
    [ 36] "IST (GMT+00:00) Dublin",
    [ 37] "BST (GMT+00:00) Edinburgh",
    [ 38] "WEST (GMT+00:00) Lisbon",
    [ 39] "BST (GMT+00:00) London",
    [ 40] "GMT (GMT+00:00) Monrovia",
    [ 41] "UTC (GMT+00:00) UTC",
    [ 42] "CEST (GMT+01:00) Amsterdam",
    [ 43] "CEST (GMT+01:00) Belgrade",
    [ 44] "CEST (GMT+01:00) Berlin",
    [ 45] "CEST (GMT+01:00) Bern",
    [ 46] "CEST (GMT+01:00) Bratislava",
    [ 47] "CEST (GMT+01:00) Brussels",
    [ 48] "CEST (GMT+01:00) Budapest",
    [ 49] "CEST (GMT+01:00) Copenhagen",
    [ 50] "CEST (GMT+01:00) Ljubljana",
    [ 51] "CEST (GMT+01:00) Madrid",
    [ 52] "CEST (GMT+01:00) Paris",
    [ 53] "CEST (GMT+01:00) Prague",
    [ 54] "CEST (GMT+01:00) Rome",
    [ 55] "CEST (GMT+01:00) Sarajevo",
    [ 56] "CEST (GMT+01:00) Skopje",
    [ 57] "CEST (GMT+01:00) Stockholm",
    [ 58] "CEST (GMT+01:00) Vienna",
    [ 59] "CEST (GMT+01:00) Warsaw",
    [ 60] "CET (GMT+01:00) West Central Africa",
    [ 61] "CEST (GMT+01:00) Zagreb",
    [ 62] "EEST (GMT+02:00) Athens",
    [ 63] "EEST (GMT+02:00) Bucharest",
    [ 64] "EET (GMT+02:00) Cairo",
    [ 65] "CAT (GMT+02:00) Harare",
    [ 66] "EEST (GMT+02:00) Helsinki",
    [ 67] "EEST (GMT+02:00) Istanbul",
    [ 68] "IDT (GMT+02:00) Jerusalem",
    [ 69] "EEST (GMT+02:00) Kyiv",
    [ 70] "SAST (GMT+02:00) Pretoria",
    [ 71] "EEST (GMT+02:00) Riga",
    [ 72] "EEST (GMT+02:00) Sofia",
    [ 73] "EEST (GMT+02:00) Tallinn",
    [ 74] "EEST (GMT+02:00) Vilnius",
    [ 75] "AST (GMT+03:00) Baghdad",
    [ 76] "AST (GMT+03:00) Kuwait",
    [ 77] "FET (GMT+03:00) Minsk",
    [ 78] "EAT (GMT+03:00) Nairobi",
    [ 79] "AST (GMT+03:00) Riyadh",
    [ 80] "IRDT (GMT+03:30) Tehran",
    [ 81] "GST (GMT+04:00) Abu Dhabi",
    [ 82] "AZST (GMT+04:00) Baku",
    [ 83] "MSK (GMT+04:00) Moscow",
    [ 84] "GST (GMT+04:00) Muscat",
    [ 85] "MSK (GMT+04:00) St. Petersburg",
    [ 86] "GET (GMT+04:00) Tbilisi",
    [ 87] "MSK (GMT+04:00) Volgograd",
    [ 88] "AMT (GMT+04:00) Yerevan",
    [ 89] "AFT (GMT+04:30) Kabul",
    [ 90] "PKT (GMT+05:00) Islamabad",
    [ 91] "PKT (GMT+05:00) Karachi",
    [ 92] "UZT (GMT+05:00) Tashkent",
    [ 93] "IST (GMT+05:30) Chennai",
    [ 94] "IST (GMT+05:30) Kolkata",
    [ 95] "IST (GMT+05:30) Mumbai",
    [ 96] "IST (GMT+05:30) New Delhi",
    [ 97] "IST (GMT+05:30) Sri Jayawardenepura",
    [ 98] "NPT (GMT+05:45) Kathmandu",
    [ 99] "ALMT (GMT+06:00) Almaty",
    [100] "BDT (GMT+06:00) Astana",
    [101] "BDT (GMT+06:00) Dhaka",
    [102] "YEKT (GMT+06:00) Ekaterinburg",
    [103] "MMT (GMT+06:30) Rangoon",
    [104] "ICT (GMT+07:00) Bangkok",
    [105] "ICT (GMT+07:00) Hanoi",
    [106] "WIB (GMT+07:00) Jakarta",
    [107] "NOVT (GMT+07:00) Novosibirsk",
    [108] "CST (GMT+08:00) Beijing",
    [109] "CST (GMT+08:00) Chongqing",
    [110] "HKT (GMT+08:00) Hong Kong",
    [111] "KRAT (GMT+08:00) Krasnoyarsk",
    [112] "MYT (GMT+08:00) Kuala Lumpur",
    [113] "WST (GMT+08:00) Perth",
    [114] "SGT (GMT+08:00) Singapore",
    [115] "CST (GMT+08:00) Taipei",
    [116] "ULAT (GMT+08:00) Ulaanbaatar",
    [117] "CST (GMT+08:00) Urumqi",
    [118] "IRKT (GMT+09:00) Irkutsk",
    [119] "JST (GMT+09:00) Osaka",
    [120] "JST (GMT+09:00) Sapporo",
    [121] "KST (GMT+09:00) Seoul",
    [122] "JST (GMT+09:00) Tokyo",
    [123] "CST (GMT+09:30) Adelaide",
    [124] "CST (GMT+09:30) Darwin",
    [125] "EST (GMT+10:00) Brisbane",
    [126] "EST (GMT+10:00) Canberra",
    [127] "ChST (GMT+10:00) Guam",
    [128] "EST (GMT+10:00) Hobart",
    [129] "EST (GMT+10:00) Melbourne",
    [130] "PGT (GMT+10:00) Port Moresby",
    [131] "EST (GMT+10:00) Sydney",
    [132] "YAKT (GMT+10:00) Yakutsk",
    [133] "NCT (GMT+11:00) New Caledonia",
    [134] "SBT (GMT+11:00) Solomon Is.",
    [135] "VLAT (GMT+11:00) Vladivostok",
    [136] "NZST (GMT+12:00) Auckland",
    [137] "FJT (GMT+12:00) Fiji",
    [138] "PETT (GMT+12:00) Kamchatka",
    [139] "MAGT (GMT+12:00) Magadan",
    [140] "MHT (GMT+12:00) Marshall Is.",
    [141] "NZST (GMT+12:00) Wellington",
    [142] "CHAST (GMT+12:45) Chatham Is.",
    [143] "TOT (GMT+13:00) Nuku'alofa",
    [144] "WST (GMT+13:00) Samoa",
    [145] "TKT (GMT+13:00) Tokelau Is."
]
```