GDA-5833 -> Remove dependency on org.apache.commons.httpclient

It has been replaced with org.apache.httpcomponents.httpclient and org.apache.httpcomponents.httpcore
GDA-5000 Switch to SLF4J 1.7.5
GDA-5000 / GDA-5780 / GDA-5771 Switch uk.ac.gda.core to Logback 1.1.0

* Update RedirectableFileLogger & ElogAppender following introduction of
  ILoggingEvent interface

* Add layout field to ElogAppender because AppenderBase no longer has one
GDA-5000 / GDA-5769 Change uk.ac.gda.client to depend on new SLF4J bridge JARs
GDA-5000 / GDA-5769 Update uk.ac.gda.core PyDev path to use new SLF4J/Logback JARs