Added code that controls voltage sources.

This code was initialy implemented for psi46expert_bare project.
Now, all code related to the voltage source manipulation will be a part of
this standalone project.
- BaseConfig.h, BaseConfig.cc and ConfigParameters.h: coniguration
  parameters for the program with possibility to store/load them to a text
  file.
- IVoltageSource.h: interface for all voltage source control classes.
- FakeVoltageSource.h: definition of fake voltage source for test purposes.
- GpibStream.h and GpibStream.cc: classes that provides interface to work
  with GPIB interface.
- serialstream.cc and serialstream.h: classes that provides interface to
  work with serial port.
- exception.h: definition of base exception class for the project.
- date_time.cc and date_time.h: functions to work with date and time.
- log.cc and log.h: implementation of logging system with different
  verbosity level and possibility to store log in files.
- units.h: definition of physical units that are used in the project.
- Keithley237.cc, Keithley237.h, Keithley237Internals.cc and
  Keithley237Internals.h: classes that provide interface to
  manipulate Keithley 237 over GPIB interface.
- Keithley6487.cc and Keithley6487.h: class that provvides interface to
  manipulate Keithley 6487 over serial interface.
- ThreadSafeVoltageSource.cc and ThreadSafeVoltageSource.h: definition of
  class-wrapper that provides thread safety while manipulating with voltage
  source.
- VoltageSourceFactory.cc and VoltageSourceFactory.h: create voltage
  source manipulation class based on configuration parameters.