NLog 4.3.0-rc3

This package has no dependencies.

RC3:
- Fix writing log files to root dir
- Autoload NLog.config in assets folder on Android

RC2:
- fix ${callsite} when code got inlined (was broken in 4.3)
- Check for TimeToSleepBetweenBatches in AsyncWrapper

NLog 4.3:

See http://nlog-project.org/2016/03/23/nlog-4-3-0-RC-is-now-available.html

# Features
- Support Windows Phone 8, Xamarin Android and Xamarin iOS (beta)
- Consistent handling of exceptions (BEHAVIOUR CHANGE)
- Control of exception throwing for configuration errors
- API improvements
- Relative paths for fileTarget
- InternalLogger: write to System.Diagnostics.Trace
- Mail Target: Allow virtual paths for SMTP pickup
- EventTarget: option to set the max length of the message and action (discard, split, truncate)
- MethodCallTarget: allow optional parameters in called methods.
- ConsoleTarget: Regex cache is instead of compiled regex, for better memory usage. This is configurable.
- Database target: don't require "ProviderName" attribute when using `<connectionStrings>`
- RegistryLayout: Support for layouts, RegistryView (32, 64 bit) and all root key names (HKCU/HKLM etc)
- Allow to free CallContext in `MappedDiagnosticsLogicalContext`
- LogFactory: Add generic-type versions of `GetLogger()` and `GetCurrentClassLogger()`
- Added `Logger.Swallow(Task task)`
- Unused targets will be logged to the internal logger
- Config classes are now thread-safe.
- InternalLogger: improved logging of exceptions (analogous to normal Logger)
- More logging to the internal logger (e.g. Async wrapper and buffer wrapper)
- Added timestamp options for the internal logger:
- Read nlog.config from Android asse

# Bug fixes
Among others:

- `${callsite}` works now for async methods!
- A **lot of** Filetarget bug fixes regarding with archiving, locking and concurrent writing.
Most noticeable:
  - Use last-write-time for archive file name. This is far more stable.
   In the past there were some issues with unexpected archive filenames.
  - Fix: archiving won't work when a there is a date in the filename
  - Fix: archiving not working properly with AsyncWrapper
  - Fix: Footer for archiving
  - Fix: Crashes with relative path without `${basedir}`
  - Fix: Archive files are never created when there are lot of write in log file
- NetworkTarget: fix possible deadlock
- Fix autoreload nlog.config with parent configs.
- WebServiceTarget: Fix HTTP GET protocol
- Bugfix: Internallogger creates folder, even when turned off
- Fix possible Nullref in `${variable}`
- `${processtime}`: incorrect milliseconds formatting
- `${processtime}`: fix incorrect negative time (rounding issue)