[RFC] Macro-based log utility for Neovim

I find logs very useful for quick debugging while developing. Vim already implemented some ad-hoc logging in some places (see `regexp_nfa.c`, `time_msg()`...). I think that having a generic logging utility is a good idea.

Use cases:
 - Log warnings about programming errors, use of deprecated functions...
 - Log informational messages that give the programmer a clue about the progress of execution
 - Help debugging the MsgPack-RPC server by logging each request
 - Quick debug by temporarily adding `DLOG`s to code
 - ...

How the log look like:

```
$ tail -f ~/.nvimlog
2014/04/28 17:15:15 [info] 4829 - Starting Neovim main loop.
```

## Main commit message

This commit introduces 4 macros (for different log levels) that can be used
to log messages to $HOME/.nvimlog:

 - DLOG: log a debug message (e.g. `DLOG("sum(%d, %d): %d", x, y, sum(x, y));`)
 - ILOG: log some useful information (e.g. `ILOG("Main loop started")`)
 - WLOG: log a warning (e.g. `WLOG("Command not found: %s", command)`)
 - ELOG: log an error (e.g. `ELOG("Out of memory. Exiting.")`)

All these macros are disabled if `NDEBUG` or `DISABLE_LOG` is defined. This
guarantees that a `Release` build won't log anything.

`MIN_LOG_LEVEL` can be defined to reduce the verbosity of the log. The log
levels are:

```
DEBUG_LOG_LEVEL     0
INFO_LOG_LEVEL      1
WARNING_LOG_LEVEL   2
ERROR_LOG_LEVEL     3
```

`MIN_LOG_LEVEL` is 0 by default enabling all levels. If `MIN_LOG_LEVEL` is set
to 2, for example, only warnings and errors will be logged.