Elektron Logging library (#16)

switch to logrus for logging.

replaced old logging library with a wrapper around logrus.
We now just need to use the exported Log(...) and Logf(...) from the logging/
package that wraps around a set of loggers constituting a chain (following COR).
Loggers are configured using a YAML file that specifies the following.
1. enabled/disabled
2. whether the message should be logged on console.
3. filename extension.
4. minimum log level.

Retrofitted source code to now use the updated logging library.
Updated the documentation with information regarding the specification
of the log config file.

Currently, the log format in the config file is not adhered to. This is going to be
addressed in a future commit.

docs/data/withSpsEnabled/SchedulingPolicySwitchTrace.md

@@ -8,7 +8,7 @@ The logs are written to a file named _\<logFilePrefix\>\_\<timestamp\>\_schedPol
 
 The format of the data logged is as shown below.
 ```
-<yyyy/mm/dd> <hh:mm:ss> <sched policy name>
+[<loglevel>]: <yyyy-mm-dd> <hh:mm:ss> Name=<sched policy name>
 ```
 
 _Note: The names of the scheduling policies can be found [here](https://gitlab.com/spdf/elektron/blob/master/schedulers/store.go#L14)_