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

When use `%Z` for date formatting, it does not always the same time as it be parsing from. That is why in some situation it complains '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."
]
```