Move command struct definitions to interface.h

Make g_commands accessable via interface.h for further references
Add help command
Add command summary for help command
Add uimage command to get cover image of a track or album
Check that the array index isn't out of bounds before trying to access it
Merge branch 'command-uri-image' of https://github.com/xemle/spop

Conflicts:
	src/interface.c
Make logging thread-safe

The `spop_log_handler()` function is not thread safe. This is especially
noticeable when using a log file, as in this case the calls to
`g_io_channel_*()` will sometimes cause other error messages to be
displayed, which will cause the log handler to be called, modifying
`log_line->str` and `log_line->len` while there are still used, causing
the following message to be displayed:

    g_string_erase: assertion 'pos + len <= string->len' failed

Closes #31, #9.