Use Parsedown instead of PHP-Markdown and PSR compliance

[ParseDown](http://parsedown.org/) is a new markdown parser for PHP which support classic markdown syntax and markdown-extra syntax too.

I did benchmark both [ParseDown](http://parsedown.org/) and [PHP-Markdown](http://michelf.ca/projets/php-markdown/) on a fresh Pico install, targeting the default homepage and ParseDown looks faster.

I used [Siege](http://www.joedog.org/siege-home/) with the following options :
`siege -b -c5 -t1M http://localhost/Pico/`
`-b` means benchmark which does not wait between requests
`-c` is the number of concurrent users
`-t` is the benchmark time. I ran it for 1 minute.

I restarted php-fpm and nginx before each siege session.

|                           |   ParseDownExtra  |   MarkDownExtra   |
|---------------------------|-------------------|-------------------|
| Transactions              |   936 hits        |   702 hits        |
| Availability              |   100.00 %        |   100.00 %        |
| Elapsed time              |   59.90 secs      |   59.18 secs      |
| Data transferred          |   2.19 MB         |   1.64 MB         |
| Response time             |   0.32 secs       |   0.42 secs       |
| Transaction rate          |   15.63 trans/sec |   11.86 trans/sec |
| Throughput                |   0.04 MB/sec     |   0.03 MB/sec     |
| Concurrency               |   4.97            |   4.97            |
| Successful transactions   |   936             |   702             |
| Failed transactions       |   0               |   0               |
| Longest transaction       |   0.60            |   1.37            |
| Shortest transaction      |   0.18            |   0.32            |

then I ran it again for 5 minutes and get the following results :

|                           |   ParseDownExtra  |   MarkDownExtra   |
|---------------------------|-------------------|-------------------|
| Transactions              |   2500 hits       |   1958 hits       |
| Availability              |   100.00 %        |   100.00 %        |
| Elapsed time              |   179.66 secs     |   179.85 secs     |
| Data transferred          |   5.84 MB         |   4.57 MB         |
| Response time             |   0.36 secs       |   0.46 secs       |
| Transaction rate          |   13.92 trans/sec |   10.89 trans/sec |
| Throughput                |   0.03 MB/sec     |   0.03 MB/sec     |
| Concurrency               |   4.99            |   4.99            |
| Successful transactions   |   2500            |   1958            |
| Failed transactions       |   0               |   0               |
| Longest transaction       |   2.36            |   2.49            |
| Shortest transaction      |   0.24            |   0.33            |