Use REGEXP_DEBUG to enable log-based regexp debugging

ENABLE_LOG is too broad to be used only to enable logging in regexp_nfa.c. Use
REGEXP_DEBUG instead.

ENABLE_LOG was defined by checking REGEXP_DEBUG in the first place.
Macro-based log utility for Neovim

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.

That's how the log looks like:

```
     DATETIME   LOG_LEVEL  FUNCTION  LINE  PID      FORMATTED MESSAGE

2014/05/01 23:46:14 [info @ main_loop:582] 44376 - Starting Neovim main loop.
2014/05/01 23:46:31 [info @ main_loop:582] 44400 - Starting Neovim main loop.
```
Introduce os_localtime_r() and os_get_local_time()

Replace localtime() with os_localtime_r() in `eval.c` and `undo.c`.
Merge pull request #644 from philix/log

Macro-based log utility for Neovim
coveralls upload should not fail the CI build

- swallow error code
Replace lalloc() with xmalloc()
No OOM error in make_filter_cmd()
Delete the lalloc() function
update CONTRIBUTING.md
Update CONTRIBUTING.md with current info

- Finish what @ashleyh started.
- Add section for Getting Started with Neovim.
- Add section for commit messages.
- Trim down on wording to keep it short.
replaced some mch_stat() with os_file_exists()
use os_getperm() in f_getfperm()
implemented os_file_get_size()
implemented FileInfo struct

This struct is a wrapper around `uv_stat_t` to hide the stat information
inside `src/os/`.
The stat file attribute will be private after all refactorings concerning
file informations are done.
replaced mch_fstat() with os_file_get_info_fd()
replaced some mch_stat() with os_get_file_info()
replaced some mch_lstat()
replaced mch_stat() in buffer.c
replaced last mch_stat() and mch_lstat()

files: memline.c and fileio.c
removed mch_lstat() and mch_stat() macros