Removed a servatrice memleak

Fix #259

Should help make valgrind’s log a bit cleaner

I should probably bust this out to a method.

Cherry-picked b9cb61abd05f83c67740aafa5e1e6939143547e4

Picurl handling

Added section on memory management to CONTRIBUTING.md.

Fix #258: Dashes to spaces.

Add icon to oracle

Added style guide.

add success notification to oracle; fixes #134

Handle PNGs (and other image formats)

Like M15’s .png that cockatrice wrongly saved as .jpg
Additionally, move the QImageReader declaration outside of the inner
loop

Removed the QLists for determining image format. Instead, using QImageReader, both when downloading/saving and when loading, to determine the correct format (Cockatrice now supports all QImageReader-supported formats).
Image loading still uses one for loop to iterate through the QList of paths to folders in which to search for images.

Refactored the magic bytes into two QLists: one containing a list of QStrings representing the supported extensions, the other containing a list of QByteArrays representing the magic bytes in hex.
Refactored the image loading loop to be two nested for loops: the outer loop iterating through the QList of paths to folders in which to search for images and the inner loop iterating through the QList of supported extensions.

Switched back to using QFile instead of QImage for saving as QImage's save function adds overhead to the file size, leaving me unsure whether the original image is affected.

Fix compilation warning introduced in #246

/cockatrice/src/main.cpp:162:16: warning: format string is not a string
literal (potentially insecure) [-Wformat-security]
        qDebug("Could not create " +
settingsCache->getPicsPath().toUtf8() + "/CUSTOM. Will fall back on
default card images.");

Added schema for cards.xml and tokens.xml in xsd format

Fix #248: the token dialog generates cards irrespective of their case and punctuation

Picture path/CUSTOM folder creation

Checks if CUSTOM folder was successfully created. If not, prints qDebug message and proceeds as normal except uses default card images instead of custom ones.

Added better support for custom card images

Add back support for custom picture urls in cards.xml